Materials for the Study of Variation Treated with Especial Regard to Discontinuity in the Origin of Species

In the G-rayed specimens there is thus a ventral trivium and a 'dorsal trivium.' (There were 2 Polian vesicles in 3 specimens, 3 in one and one in the other, but in the normal also these vary in number.) The stone-canal was single in all ; but in one of them it could be seen that the canal arose in the interradius to the left of that which bore the madreporic plate, suggesting that the radius thus crossed was supernumerary ; for in a normal the interradius of the dorsal mesentery is in the centre of the bivium. In a normal there are in the calcareous ring two radials on either side between the dorsal mesentery and the ventral median radius. In 4 of the abnormals (to which alone what follows refers) there were 3 such radii on the left and 2 on the right, while in the 5th specimen there were 3 on the right and 2 on the left.

The respiratory trees of the normal are in the right interradius of the bivium and in the left interradius of the trivium. In the 6-rayed they are in the left interradius of the ventral trivium and in the lower riidit interradius of the dorsal trivium, agreeing with the normal and shewing t hat the right radius of the ventral trivium is not an intercalated one. Next, the mesentery in its course traverses in the 6-rayed form I radii and '.'> interradii, the lower right interradius of the dorsal trivium with its L' adjacent radii alone being free. In the normal, 3 radii and 1* interradii are thus traversed, the right bivial interradius and its 2 adjacent radii being free. Therefore the right radius of the dorsal trivium and of the ventral trivium are not intercalated. The central radius of the ventral trivium has already been excluded; therefore the intercalated segment is either the middle or the left of the dorsal, or the left of the ventral trivium.

In a normal, the mesentery which is attached to the alimentary canal at that place \\here its upward portion again turns downwards comes from that interradius which bounds the ventral trivium on the left. This is the case also in the abnormals, and therefore the left radius of the ventral tr'muin is not intercalated. Of the two remaining radii the left of the dorsal trivium is in nowise abnormal, but the central dorsal radius IS abnormal in that it is crossed by the sand-canal, therefore the central ilm-aul is the intercalated radius.

And since in four cases there were three radii in the calcareous ring on the left, between the interradius of the stone-canal and the central of the ventral trivium, and two on the ri<//tt, therefore the new .-.cement j.s in them intercalated on the l-ft of the median interradius of the bivium ; while in the iifth specimen the intercalation has been made on the /•/<//,/ of the same interradius. Now all this argument rests on the premiss that the several members of a series of differentiated parts cannot undergo a Substantive Variation in correlation with Meristic change in the total number of members constituting the series. It is assumed that there can be no redistribution of differentiation.

This assumption has now in many cases of Linear Series been shewn to be false. To refer to one of the simplest cases, there is, in the case of the Frog, evidence that the peculiarities of the 9th vertebra may be wholly or in part transferred to the 10th vertebra, when by Meristic Variation there are 10 vertebra? (Nos. 56, 57 and 60), and the like has been shewn in many other examples (cp. No. 35). The functions (as indicated by the structures) of the vertebra? may be redistributed on the occasion of Meristic Variation.

Will anyone affirm that similar redistribution of differentiation may not happen in the Meristic Variations of Echinoderms ? 543 Variations in organs of Holothurioidea. LAMPEBT calls attention to the great variability found in this group and the consequent difficulty in distinguishing specific characters from individual abnormalities. These variations often take the form of alterations in the number of organs. For example, the distribution of the tube-feet is liable to great alterations during the lifetime of individuals. In some forms (as Thyone and Thyonidium) the feet are confined to the ambulacral areas in the young animal, but are distributed over the whole body in more mature individuals ; and in species of the genus Stichopus, though the tube-feet are arranged in rows, yet in old individuals this arrangement may become obliterated. On the contrary, in others, as for example, Holothuria graeffei, the arrangement of the feet in thoroughly mature specimens is still most sharply denned.

The number of the tentacles is generally a multiple of rive, and such cases as Amphicychis and Phyllophorus in which other numbers are found, are rare. In these forms the tentacles are said to vary both in number, position and size, but the number is always about 20. The case of Thyonidium molle is cited as an extreme case. Of this species 4 specimens had 20 tentacles arranged in a paired manner as in typical Thyonidia ; other specimens had 20 tentacles of similar length ; others had from 16 to 19 tentacles of nearly equal lengths, and others again had from 19 to 21, which instead of being disposed in pairs were placed irregularly, some being larger and some smaller.

Of all the organs, the Cuvierian organs are the most variable and they are of little value for purposes of classification. Their number is very inconstant and they may even be absent altogether. It is impossible to distinguish any circumstances whether of locality or of structure in which the individuals without Cuvierian organs differ from the others which possess them. The two chief appendages of the water vascular ring, namely Polian vesicles [cp. No. 642] and the stone-canal are usually constant when they are single, but in rare cases there are exceptions even to this rule. If however more than one of these organs is normally present, it may generally be assumed that there is no constancy in their numbers, and in such cases the number of the Polian vesicles is especially variable. A few species have been recorded in which, from a single Polian vesicle, secondary ones are formed by lateral outgrowths.

The calcareous plates are of all the organs the least liable to variations, but in certain cases they are stated to change with age. Variation from the pentamerous condition has been many times observed, though considering the vast number of specimens collected it must !.»• a r;ire occurrence. In Tetracrinus the four-rayed condition is normal, and it is an especially interesting circumstance that in this fin-Hi Variation to Imt h a I'm- rayed and to a three-rayed condition has In i 11 observed. Fur nearly all the references to the following facts I am indebted t<> tinuseful collection of evidence on the subject given by BATHER, F. A., Quart. Jour. <;>ol. 6'oc., 1889, p. 149.

Fntir-rai/ed varieties of five-rayed forms1. (544. Holopus rangi. This genus was originally described from a Subsequently, ~> rayed examples were obtained and this condition was Among many hundreds of calyces in Brit. Mus. one only is 4 rayed, BATIIKK, /. <•., p. 1 55. G4G. E. nutans : I rayed specimen at Tiibingen figured in QUENSTEDT'S dorf .Mus., Hour. ( Itather). »!47. E. caryophyllatus : 4-rayed specimen seen at Stuttgart. Such a specimen |Mhe same] Uosixus, Tentaminis de Lithozois . . . Prodr. Arc.,

Poppelsdorf Mus. (Bather). G4S. Pentacrinus : a 4-rayed stem-joint from the Chalk, MANTELL, G. 4 sides, one being quite flat. This fiat side had an articulation for a 6 :>.S. Balanocrinus subteres : 4-sided stem quite regular, ibid. G54. &• bronni : "the articular surface shows 4 sectors quite regularly disposed ; this peculiar character is continued over the whole series of 658. Actinometra paucicirra : 4-rayed specimen, Hid. " In call these [Nos. 657 and 658] the anterior ray (A) is missing, so that the mouth, instead of being radial in position is placed interradially between the rays E and B." CARPENTER, I. c.

Compare the following case of imperfect change towards the 4-rayed state : 659. Cupressocrinus crassus : abnormal calyx (now referred to this species, see BATHER, I. c., p. 169) has one segment of the calyx reduced in size and bearing no radial plate or arm. This reduced segment is covered in by the adjacent segments so that the calyx as a whole is regularly 4-sided. GOLDFUSS, Nova Acta Ac. C. L. C., 1839, xix. p. 332, PI. xxx., tigs. 3 a and b [cp. No. 665].

660. Actinometra pulchella : doubtful case of six rays, CARPENTER, 661. Antedon sp. Six-rayed specimen. "The additional ray is inserted between the two of the right side (D and E)." CARPENTER, I. c. 662. Rhizocrinus lofotensis : 6-rayed specimen. Four and six rays stated to be more common in Rhizocrinus than in other recent Crinoids ; seven rays are also found, but very rarely. In Pentacrinus no 6-rayed specimen seen. CARPENTER, P. H., Chall. Rep., xi. Pt. xxxii. p. 38, PI. vin. «, figs. 6 and 7.

*663. Pentacrinus jurensis (probably) : stalk with 6 sides. [Fig. represents two adjacent lobes of the stalk as smaller and closer together than the rest, suggesting that perhaps these two may correspond with one lobe of the normal.] DE LORIOL, I.e., PL CXLIV. fig. 7. 664. P. jurensis : 6-sided stalk having two adjacent lobes larger than the others, ibid., fig. 10. The following is a case of imperfect change towards the six-rayed state : 665. Sphaerocrinus geometricus : abnormal specimen having the basal plate irregularly six-sided by reason of the flattening of the external angle of an infra-basal piece. Three of the sides are normal and each of these bears a normal parabasal ; but of the other three sides two are rather shorter than the normal sides and each of them bears a somewhat smaller parabasal. Upon the sixth side between these two, is a still smaller parabasal. The radials are five as usual, but one of them articulates with the smallest parabasal and in connexion with this its form is changed [for details see original figure]. Sculpture, <fec. normal. ECK, H., Verh. naturh. Ver. preuss. Rheinl., 1888, Ser. 5, v. p. UQ,fig.

l-'i.i. !.".!. I. Niitinal four-rayed basal of Tetracrinus moniliformis (from Birmens- II. A tin.' rayed basal of the same species from the same locality as I. III. A tive-ray<d basal of the same species from Oberbuchsitten. *6G7. Cupressocrinus gracilis. This form has normally a 5-rayrd calyx, and a ">-sidrd basal plate containing only 4 canals round the central canal ( Fig. 132, I). Varieties have been seen in which Fir;. 132. Cupresmcrinn* iinn-ilix. The normal form of the basal is shewn in I. A fnrin with live canals round the central is represented in II, and in the specimen shewn in III there are three peripheral canals. See No. 667 </.

668. Abnormalities in the manner and frequency of branching in the arms of Crinoids branches in t\v.> arms symmetrically placed with regard to the axis. BATKSON, W., /'. /. g., is'.in. p. r.sl. !i^. I (in, w in Coll. Sur-;. Bias.). The abnormal arms were //, and <•, of the usual nomenclature, as shewn in Fig. 133. For details see original description. 670. Symmetrical change in number of rays is common in some of the forms. Asterias rubens and A. glacialis are frequently seen with 6 or with 7 arms symmetrically arranged, and I have

FIG. 133. Antcdon rosacea having two arms abnormally divided. The figure A shews the relations of the two abnormal arms, &„ and t'j, to the mouth and anus. B shews the arm b.2. (From Proc. Zool. Soc.) seen one with X. Individuals with 4 arms occur, but are much less common tli.-m those with 6. I have seen Asterina gibbosa with 4 ray>, and a -prrimeii (Scilly) given me by Mr S. F. Harmer has ii rays, of which 2 are a little nearer together than the others (su^.'-tin^ division of ,-i ray). Mr K. \V. MaeBridr tells me that In- has sc.-ii st -\i-ral 6-rayed specimens of this species. Mr E. .). Bl'-s kindly tells me that he dredged a 4-rayed Porania pulvillus in the Clyde estuary. There appeared to be no trace of a tilth ray and the specimen was as nearly as possible symmetric:! I.

G73. /''irfiiif il'u-ixiun of an arm is fairly common in Asteroids, but less common I believe than the total variation in number, though I know no statistics on this point. For a figure of Asterias (Hippasterias) equestris L. with a bifid arm, presenting no appearance as of regeneration see TIKDKMANX, Zdtxchr. f. Phys., 1831, iv. p. 123, Plate 1. The two following are peculiar cases. 6 / 4. Cribrella oculata : one of the arms bearing a branch, not as a radius, but about (in dried specimen) at right angles to the normal arm, the property of Prof. C. STEWART, who kindly shewed it to me.

675. Porania pulvillus, Cray (a Starfish): Specimen 5 cm. in diameter, having five short rays. The ray opposite the madreporite u-Jien viewed from the, aboral surface is seen to be distinctly bifurcated at Fio. 134. Pitrniii'i jiuli-illnx, No. 675, having the arm opposite the madreporite abnormally divided as shewn at x and y. (From a sketch kindly sent by Prof. about 1 cm. from its termination. The ambulacra! groove of (Fig. 134) this abnormal ray divides into two branches at a distance of 2 cm. from the edge of the mouth. One of these branches runs along one of the forks of the ray to its extremity without further complication ; but the other branch, belonging to the second fork, divides again 2 mm. from the first bifurcation, so as to form two tracts which unite with one another 3 mm. further on, thus inclosing a small piece of the ordinary integument in an ambulacral area. Finally, this anibulacral area divides once more close to the tip of the ray. There are no signs of injury or disease in the specimen. HERDMAN, W. A., Nature, 1886, xxxi. p. 596. [I am indebted to Professor Herdman for the accompanying diagram of this specimen.]

In the Echinoids there are (1) cases of total Variation to a 4-rayed form with 4 ambulacra and 4 interambulacra1 : (2) cases of partial or total disappearance of a definite ambulacrum or interambulacrum, which can be named either because part of it is present, or because two sets of similar plates thus become adjacent: (3) a case of total variation to a 6-rayed form : (4) cases of imperfect reduplication of a radius, thus forming an imperfectly 6-rayed form.

Fie. 135. Cidarites coronatusf No. 670, a regularly 4-rayed specimen from oral surface. (From A. B. Meyer.) *677. Echinoconus (Galerites) subrotundus : 4-rayed specimen in ' Woodwardian Mus. (Fig. 136). The ambulacral and interambulacral areas are relatively wider than in a normal of the same size, the space of the areas that are wanting being as it were shared among those that are present. Apical disc roughly rectangular, and seems to be composed of 4 perforated basals (genitals) and 4 perforated radials (oculars). The basal plate corresponding to the posterior unpaired interambulacral area is perforated, though normally ituperforate. Statement made that

, Arch, de Biol., 1891, xi. p. 632, says that Echinoconus vulgaris has been seen with only three radii, but no authority is given. the parts missing are those which lie on the left side of a line drawn through the middle of the anterior single ambulacrum and the posterior 1. View of apical system. 2. Seen from side. 3. From apex. 4. From below. iia, anterior ambulacrum ['?]. mp, madreporite. «/, anal interradius. The parts are lettered after Itoberts.

unpaired interamlmlacrum, hut it is not possible to say which of the ]i;iirnl areas of this side are wanting, as the pores in the ambulacra! plates round the peristome are indistinctly shewn. ROBERTS, T., Geol. -I/".'/., 1S91, Dec. in., vin. p. 116, figs. ij7S. Discoidea cylindrica : a -1 -rayed specimen, absolutely symiiirtri<'al. There are only 4 oculars corresponding with the 4 ambulacra. COTTEAU, G., PaLfrang., 1862—67, vn. p. 31, PI. 1011, figs. 6 and 7. [This is exactly like KOBKKTS' case No. 677 and is illustrated by beautiful figures (q.i'.). Cotteau in describing it says that the anterior ambulacrum is wanting. It is difficult to see any sufficient reason for the determination that this ambulacrum in particular is u anting. For in this case there are only 4 sets of interambulacral plates as well as 4 ambulacral areas in perfect symmetry. The anus of course lies between two ambulacra ; and as the whole number is even and the radii are symmet rieally arranged, there is thus no ambulacrum in (lie plane of the anus. Hence the suggestion that it is the anterior ambulacrum which is wanting. But if by Variation an Echinid has 4 symmetrical radii it would always seem that the

anterior ambulacrum was missing, whether it be the anterior ambulacrum, or the left anterior, or the left posterior that is wanting, or even if all 4 new ambulacra correspond with all 5 of the normal.] (2) Partial or total disappearance of a definite ambulacrum or '680. Echinus melo, having only four complete ambulacral areas (Fig. 137). The specimen is not spherical, for the apical system is warped over in one direction and the oral pole is pulled in an opposite direction, while the shell is much higher in the region of the apical system than it is at the opposite side. There are only four ocular plates, which are subequal, the madreporic plate and the plate opposite to it being somewhat larger than the other two. The genital plates are also four. Only four ambulacral areas leave the apical system, and at that point they are almost symmetrically disposed. Lower down however a triangular series of plates bearing ambulacral pores is intercalated between the plates of one of the interambulacral systems which it divides into two. This intercalated series is of course the representative of the ambulacral area which is wanting at the apex of the shell. The Jive ambulacra are nearly symmetrically disposed round the oral surface just as the four ambulacra are round the apical system. This transition from, a tetramerous to a pentamerous symmetry is effected by complementary changes in the amount of divergence of the rays as they pass down the shell. Examination shews that the ambulacrum which is thus partially absent is the right posterior. PHILIPPI, Arch. f. Naturg., in. p. 241, Plate.

*681. Amblypneustes formosus : a 4-rayed specimen having a somewhat asymmetrical test. One of the interambulacral regions is abnormally wide, and at about 9 plates down the side of the test in this region a wedge-shaped piece composed of several partially distinct plates bearing 7 pairs of ambulacral pores. This fragment doubtless represents the deficient ambulacral area. The apical system consists of 10 plates. The two genital plates of the abnormal area are reduced in size, and the ocular plate between them is abnormally large. Considering the madreporic plate as indicating the right anterior interambulacrum, it appears that it is the left anterior ambulacrum which is thus deficient. The height of the shell at the abnormal side is less than at the other. BELL, F. JEFFREY, Jour. Linn. Soc., xv. p. 126, Plate.

In each of the foregoing the missing ambulacrum is actually at some place represented by plates of ambulacral character, and the shape of the test is greatly changed in correlation with the partial disappearance of the radius. The following cases differ, in that in them one ambulacrum is wholly wanting in the affected radius, and the interambulacra are contiguous with each other. Curiously enough in two of these specimens the symmetry is changed little or not at all. The cases in Hemiaster were all Algerian fossils '.

1 Besides those here given in the text, GAUTHIEK in the same place describes an interesting case of symmetrical reduction in the two posterior ambulacra of Hemiaster alricanus. 1 hi. 137. A'C/I/HHS wt'hi. No. 680, having the right posterior ambulacrum Initially nil-rut. ", anterior ambulacrum, ra, In, ri^'ht and left anterior ambulacra, rp, lp, ri^ht and left posterior ambulacra. I. View from apex. II. View from oral surface. (From PHILIPPI.) (582. Hemiaster batnensis : specimen in which the left posterior ambulacrum is not present, and the ambulacral groove is only indicated by a shallow depression, beyond which there are some rounded pores which continue the ambulacral area beyond the fasciole. The corresponding ocular seems to be absent. The test is of normal form, but the median suture of the right posterior interambulacrum is not quite straight. GAUTHIER, M. V., Comptes rendus de VAss. pour I'av. des sci., 1885, xm. p. 258, PL vn. fig. 1.

683. H. batnensis : very similar case of absence of right anterior ambulacrum and corresponding genital and ocular plate, ibid., fig. 3. 684. Hemiaster sp. : left anterior ambulacrum wanting and is gone without trace. There are only 4 oculars and 3 genitals. In correspondence with this variation there is considerable change in symmetry of the test, which is irregular, the anterior and right anterior ambulacra being deflected from their normal courses. [See details.] Ibid., figs. 4 and 4 bis. [Here, where there is a clear differentiation between the several ambulacra, it is doubtless possible to affirm that such a definite ambulacrum is missing, for the two interambulacra are left adjacent to each other.]

685. Echinus sphaera (0. F. Miiller): specimen described in which the left posterior interambulacral series of plates is almost entirely absent. The details of the structure are as follows : the genital plate which stands at the head of the left posterior interambulacrum is reduced in size in all directions ; but the two ocular plates which should be separated by it are somewhat enlarged, bearing several extra tubercles, and meet together peripherally to the genital plate. The series of interambulacral plates which should begin from this genital plate are represented by a rudimentary row of small tubercles : the ambulacral systems which are normally separated by these plates are consequently almost contiguous. The rudimentary interambulacral series widens somewhat at a short distance from the apical series and forms a small island of interambulacral structure bearing 4 large tubercles. Beyond this, viz. at a point placed about ^ the distance from the apex to the oral surface, the two ambulacra again unite and are continued as a single ambulacrum of double width. DONITZ, W., Mullens Arch. f. Anat. u. Phys., 1866, p. 406, PL xi.

*688. Amblypneustes griseus : having one of the ambulacra doubled (Fig. 1 .'!!') : tinapical system was normal. The width of the anterior ambulacra! region was almost double that of the others : it contained two ambulacra lyimr side by side, each, as usual, composed of a double row of plates with an ambulacral area and two poriferous zones. The I'm. 139. Anilli/pneustes griseus, No. 688. Specimen having the anterior ambulacrum doubled. I. The test seen from the apex. II. Details of anterior ambulacrum shewing combined poriferous zones between A and A. The dotted line bisects the ambulacrum of double width. (After STEWABT.)