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

FIG. 2.j. I'trimiyx excaratm. Diagrams shewing some of the variations in n-.-jM.-ct of the number and positions of the openings of the spermathecte and p-in-i;itive pores. From BKUDARD, P. Z. S., 1886. Perionyx griinewaldi, Michaelsen. Normally a pair of male l pores on the IStli segment, and a single oviducal opening fur tintwo oviducts in the middle line of the 14th segment. 107. In two specimens a dim-rent arrangement was found. One of the-c had the oviducal opening in the 15th segment [position of male openings not specified and presumably normal].

10s Tinother had two oviducal openings, one in the 13th and one in the 1 1th segment [not stated whether these openings were median or lateral, nor whether each of them was a double structure as of course tinnormal female opening is]. In this specimen the male i. Innings also were placed anteriorly to their normal position, being in the 17th segment. M icn AF.I.SKN, ./<//*/•//. <1. Hamburg, wiss. Anetalt, 1801, VIM., p. 34. A 1. 1. 1 IMS. In Terricohe generally, the £ pores are on the 15th, ami the ^ pores on the 1 1th, as in the common Earthworm.

K)!t. Allurus tetraedrus, a widely distributed form, has £ pores on the liitli and J pores on the 14th, the £ pores being thus in front of tin- J pores as a specific character. I'nder the name Allurus dubius .Michael.sen described two specimens having the male pores on the 1 1th in.-t.'ad of on the li'.th. ami the £ pores on the 15th instead of on the 1 Ith, i-adi lii-ing thus one segment in advance of its normal place [backward Honm-osis]. MICHAKLSKN, \V., Ja/trb. Hamb. u-is*. .!//>•/., l's'.'U, VII., ]>. 7; see also Arch. f. Natury., 1892, LVIII., p. '_'•"' 1 . ( 'omjiarr No. III.

II". 1'iesides these is a batch of S specimens of A. tetraedrus, loc. unknown, (! specimens had both £ and J pores in the 14th. Clitellum began in 'J.'inl, tuberc. pubert. in L'lth. These specimens are thus intermediate between A. //• /vy/m/x, which has the pores as in Lum- 111. Allurus putris : specimen having £ pores on 13th (instead of 15th) as an abnormality; in it the other external generative organs (and doubtless the internal also) were 2 segments higher than usual, the $ pore being on the 12th instead of 14th. Tuberc. pubert 26 — 28. MICHAELSEN, Jahrb. Hamburg, wiss. Anst., 1891, viu., p. 8. Compare No. 109.

112. Allurus sp. : specimen having 1. side normal; right side, <£ pore in 12th, $ in llth, clitellus and tuberc. pubert. one segment higher than usual. Ibid. *113. ENCHYTRXEID.E. </ pore generally in the 12th segment. In Buchholzia appendicuiata (Buch.) it is on the 8th, as a specific character. In Pachydrilus sphagnetorum (Vejd.) it is either on the 8th or on the 9th, according to individual variation, the other parts being then disposed as follows :

114. Perichseta hilgendorfi, n. sp. Mich. 7 specimens. Variation in number of spermathecal openings, as follows. 5 specimens had 2 pairs in the groove between segments 6/7 and 7/8 ; 1 specimen had 3 pairs, between 5/6, 6/7 and 7/8 ; 1 specimen had only one, on the left side between 6/7, which corresponded internally to a single sperrnatheca [other variations also observed in these specimens, q.v.]. MICHAELSEN, W., Arch. f. Naturg., 1892, LVIII. p. 236.

Perichaeta forbesi (an Earthworm from New Guinea). In this animal a pair of spermathecre is placed in the 8th segment and another pair in the 9th. Two specimens only have been examined and in both of these an additional spermatheca was found on the left side, internal to the other. In one individual the 5th spermatheca was in the 8th segment, and in the other it occurred in the 9th. BEDDARD, F. E., Proc. Zool. Soc., 1890, p. 65, Plate. Aiioiobophora lissaensis. Similar variation in spermathecffi, MICHAELSEN,

117. Hirudo medicinalis. The number of pairs of testes is variable, but 9 pairs most often found. Of 31 specimens of this species, 21 had 9 pairs, 6 had 10 pairs, and 4 had 9 on one side and 10 on the other. CHWOROSTANSKY, C., Zool. Am., 1886, p. 446. Hirudo officinalis : of 7 specimens, 5 had 9 pairs of testes, 1 hud in pairs, and though in the 7th specimen there were •2 pairs, th.- vas deferens of the last pair of testes ended blindly. find. Hirudo medlclnalis. Fairly often the vas defereus is prolonged beyond the

',[], ,,„! having passed through five annuli, ends in a glandular mass oi irn Case given in which the 7 last testes of right side were absent or onh :t, ,1 l-y amorphous material, the testes of the left side being abnormally *120. Hirudo officinalis : an individual having a supernumerary penis and veneula seminalis of the right side, in the 5th somite. ji\ jirni.' in normal position; p-, supernumerary penis; rx, the usual vesicula? si-miiialt^ ; /•.-•-, supi-rnumerary vesicula seminalis. (From a diagram sent to me by Mr (ribaon.)

Tiniiunnal penis in the sixth segment was fully formed and into it opened on rithrr side a vas deferens, provided with a vesicula M'lninalis as usual. But the vesicula of the right side gave otV in addition a vas dcffiTiis, which passed forwards into tli.' fourth segment and there enlarged into another vesicula M-iniiialis. This additional vesicula was connected by a duct with a supernumerary penis placed and opening in the middle of the fifth M'gnieiit. The parts of the left side as well as the female organs were normal. [I have to thank Mr Gibson for furnishing me with a diagram (Fig. 2(>) supplementing the puMi-hed account.] GIBSON, R. J. HARVEY, Xature, 1887, xxxv.,

Aulastoma gulo <Ilor>c Leech). In this form as is usual. anioii'4 the Gnathobdellidce there are from 9 to 12 pairs of testicular sacs which communicate with a tortuous vas deferens on each side which together enter a single penis. The paired ovaries are placed behind this and the oviducts unite to form *121. In a specimen found amongst a large series investigated, each vas deferens opened by a separate penis, of which the most anterior opened in the 20th annulus and the posterior in the 25th. The female apparatus was similarly divided. One ovary was placed near the penis in the 25th annulus and from it a vagina passed down to open with the penis. The other ovary, with a similar vagina, lay in the 30th annulus. ASPER, G., Zool. Am., 1878, i., p. 297.

Variation in these two groups appears in such similar modes that points of special consequence in both may conveniently be spoken of together. 1. As elsewhere seen, so here, there are forms, e.g., Perionyx excavatus or Pachydrilus sphagnetorum, shewing great variability, while others, the common Earthworm for instance, rarely vary. 2. Both forward and backward Homoeosis may occur ; a form normally having the </ pores, for instance, on the loth segment, may as an individual variation have them on the 16th (No. 105), while an individual of another genus, starting from the same normal, may have them on the 13th (No. 111).

3. As in other cases of Homceosis, when a member of a Meristic Series, in this case a segment, develops an organ proper to another segment, this organ is formed in a place serially homologous with its normal place. (To this principle certain limitations must hereafter be introduced.) 4. Variation may, or may not, be simultaneous and correlated in the several systems. The position of the $ openings, for example, may or may not vary similarly and simultaneously with that of the </ openings, though on the whole the evidence suggests that such correlation is not uncommon. The facts seen in the genus Allurus, in which one species (A. tetrdedrus) has the </ pore normally in front of the ?. pore, sufficiently indicate that the variation in the position of these two openings is not always so correlated. It may be further mentioned that variation in number of ovaries seems to occur generally without correlated variation in the number of oviducts.

5. Such Variation may or may not be simultaneous on the two sides of the body. When not thus bilaterally symmetrical there may nevertheless be a full correlation between the parts of the same side. 6. The evidence does not indicate any limit to the number of segments which may take on a certain character, or approximate to a ^i\.-n pattern. The highest number of ovaries, for , recorded, is 7 pairs : hut there is nothing to shew that aente mi^ln n«.t undergo similar Homoeosia (The provi diminution in size of these ovaries from before backwards in this case i- \\orth noticing.)

7. Tinprinciple so often manifested in the evidence of Variation, that the magnitude, completeness, and symmetry of a variation l».-:trno necessary proportion to the frequency of oreunvnee of that variation, is here strikingly exemplified. 8. 'Id'- evidence as to the existence of two varieties of r.ii-/,t/ilri!>i* .sy///'/<7//'Y/, /•/////, the one with all the organs a segment higher than their place in the other variety may be well compaivd with SiiKititi.\(;TON's observation, that in the Frog and in several Mamnial> (see Xo. 70) the individuals could be roughly divided into two classes according as the lumbo-sacral pi. -MIS was formed more anteriorly (" preaxial class") or more posteriorly ('• po-ta \ial class").

9. In the evidence as to Perionyx, it was seen that many of the arrangements found occurred in single specimens only, -i ingesting the inference that the systems do not fall into one of these conditions more easily than into others; nevertheless of each of three abnormal arrangements two examples were found, a circumstance hardly to be expected on the hypothesis of fortuitous Variation. 10. It is perhaps unnecessary to point out that the examples of Variation given are in their several degrees Discontinuous, and that by the nature of the case the Variation by which the several -peeitic forms have attained their particular numbers and characteristic disposition of organs, must almost of necessity have been thus Discontinuous.

The following facts ivspeeting Variation in Cestoda are chiefly taken from LKI < KART, Piirnsitcn des Menschen1. the variations here enumerated, abnormalities of several other kinds (\ari.-ition in number of suckers, prismatic segments, Uit'uivatioii. AC.) are known in this i^roup, l>ut as these do not directly illustrate the Variation of Linear Series, consideration of them must The degree to which the parts hearing sexual organs are -p.-u-a ted from each other differs greatly in the various groups of < 'erodes. In some (2We»//opAonw) the segmentation amounts to an inconsiderable const rid ion, while in /,i;/,,l<i the generative organs are repeated several times in a common body. L., p. 347.

1 In what follows tin.- Irttrr L. is u<ccl in reference to this work. 122. Even in the groups whose segmentation is commonly perfect, variations in the degree of separation between the proglottides are not rare. It frequently happens that specimens of Tcenia are found in which the external segmentation is partial, being only found on half of the contour. This abnormality, which does not affect the internal organs, occurs several times in the same chain. MONIEZ, R., Bull. Sci. du Nord, x., p. 200.

123. Taenia saginata. Cases of the "intercalation" of a triangular, wedge-like segment between two proglottides are recorded. In such cases the generative opening is on the same side as in an adjacent segment, not taking part in the alternation. L., p. 572. Compare with similar phenomena in Chffitopoda (p. 156). The evidence of abnormal repetition of parts occurring in single proglottides bears on the question of the relationship of the perfectly segmented forms to the less fully segmented. 124 Taenia saginata: a specimen 128 mm. long, wanting the head, without any division into segments. The longitudinal vessels were seen, but no transverse vessels were discovered. On the margins were numerous genital openings,- of which 41 were counted, each leading from a genital organ. There was no regular lateral alternation between the genital papillae, but they were disposed without uniformity of pattern, and several were closely approximated to each other. In no part was there any trace of division into proglottides. From the characters of the genital openings and from the number and size of the calcareous bodies together with other histological details, the specimen was determined without much doubt as Tcenia saginata. GROBBEN, C., Verh. zool.-bot. Ges. Wien, 1887, Bd. xxxvu., p. 679, fig.

Such repetition of the generative openings in single segments is very common, especially in Twuia saginata, and indeed examples of it may be seen in most chains of segments. Usually such repetition is confined to one segment and is not striking. Five generative papillge have been seen by L. in one segment, and COLIN [ref. not found, W. B.] described 25 — 30 genital pores in an unsegmented piece measuring 15 cm. L, p. 571. 125. Repetitions are not confined to the generative openings, but the generative organs themselves are also thus abnormally repeated. In cases in which several sets of generative organs occur in the same segment it is found that those near the middle of the segment are the least developed. In these cases, though the different organs frequently cross each other, Leuckart found no anastomoses between them, but the number of distinct sets of generative organs was the same as the number of pores.

It was not found that the length of the segments increased in the same ratio as the number of the pores they contain. For example, a segment with two pores measured 18 mm. in length 12G. Ta-nla. Case quoted by Leuckart from HELLER of a Tania having generati\. "I'l-iiinrs I'lar. <l on the turfaet of the segments. Leuckart himself has never seen an i x:mii'liof tliis variation. [Original reference not found] L., p. 570, Note. 1-J7. Taenia solium and T. saginata. Specimens are known having two generative pores opposite each other at the same level. In .such cases each leads to a male and a female duct with cirru— sac and receptaeulum seminis; but the organs for preparing the ova are normal in construction, as the two vaginae lead to a common uterus and shell-gland. Two cases only have been seen by LKTCKART and he cites another from WERNER. L., pp. 52!) and ~>7 1 .

Taenia solium in which the pores are normally alternating, may be found with symmetrically developed pores; and on tin contrary, T. elliptica in which they are normally symmetrical, may occur with anasymmetrical arrangement. L., pp. 353 and l-2'.i T. saginata: in a chain of about 6'5 metres in length, and containing some (>50 joints, there was found a single, heart - shajH-d, supernumerary joint like those described; a single joint u.i- i.>mid with two genital pores, one being on each lateral border at about tinsame level.

The largest number of consecutive joints having the genital jion-s on tinsame side was six. TUCKERMAX, F., Zool. Anz., XL, |:;o Taenia coenurus. Speeimen observed by Leuckart in which the l;i>t s or Hi segments shewed a transposition of the generative organs, those whieli usually lie at the distal end being placed at the proximal. This change of position was especially seen in the ease of organs engaged in the preparation of the ova. The proximal proglot I ides of this individual were normal. The transition segment, between these two regions contained two simple \eMcul.-e seiuinales and two marginal papilla? which were on oppu-ite sides; bm in spite of the resemblance of these structures to genital pores. neither opening, nor cirrus, nor vasa deferentia could be distinguished. L., p. 504.

I'll Amongst chains of normal proglottides it is not rare to find Speaking generally, slight abnormalities are far more common than great onea Nearly every specimen of Tapeworm has indi\idual peculiarities, and these generally repeat themselves in the same chain of proglottides. This repetition of the same abnormality in different parts of the chain is also the rule for the great,. ]• abnormalities also. L.. pp. .">:!!). 572 and 573. UNDER the general heading of Variation of branchial openings facts will be given relating to the following subjects.

I. Variation in the patterns formed by the bars, vessels and stigmata of the branchial sac in Ascidians. III. Abnormal openings in the cervical region of Mammals, known as " cervical fistula^," and external appendages called " cervical auricles," or "supernumerary ears," present sometimes in connexion with such openings. With reference to the two first subjects the evidence is only fragmentary, but the instances recorded seem to be of sufficient consequence to warrant their introduction in illustration especially of the magnitude and deh'niteness of Variation.

Variations affecting the opercular opening in Amphibia are mentioned in connexion with Bilateral Series. 132. Ascidia scabra. Branchial sac in one specimen shewing abnormal and irregular structure owino- to branching of transverse vessels. The 133. Ascidia virginea (O. F. Miiller): a case of great irregularity exactly similar to the above. Ibid., p. 330. 1 34. Ctenicella lanceplani. Branchial sac may present characters due to variations in disposition of transverse vessels &c., which assume three distinct patterns or marked varieties. LACAZE-DUTHIERS, Arch. Zool. Exp., S. 1, Vol. vi., p. 619, Vol. XXXIIL, figs. 9—11.

*1.S">. Ascidia plebeia 'Alder): branchial sac has very characteristic appfaraii. ••• and is very constant in the size of meshes, papillae &c. < >nc point is liable to variation: as a rule the transverse vessels are of tin- .-aimcalibre. but in several specimens every fourth vessel is much wider than the intervening three. HERDMAX, p. 331. i:lt!. Ciona intestinalis : meshes vary but according to no apparent method: "> stigmata in a mesh normal; 4 and 6 met with frequently; 1" tinutmost seen. HERDMAX, p. 332.

1 •'•". Ascidia aspersa. In typical specimens, transverse vessels all same size, tinmeshes being square and undivided, but individuals occur in which many (not all) of these square meshes are divided by delicate tnuis\ .-r>H vessels into pairs of oblong areas. HERDMAX, p. 332. I:;N Styela grossularia. The genus Styeta is characterized by the pn-M-iice of branchial folds, normally four on each side, but in this species tinfolds are almost obsolete, being entirely wanting on the ',< •!> -ide ami reduced to a single slight inward bulging on the right -ide. Itc.irin^ internal longitudinal bars. This fold is separated from tindor-al lamina by a broad space without internal longitudinal bars. \ -imilar wide space is present on the left side of the dorsal lamina, ami two others .>n the vertebral edge of the sac, one on each side of the

• •ndostylr. These spaces vary in size in individuals. They commonly • •ontain hi stigmata, but numbers down to 12 were frequent and in one case 1() only were present: only once more than 16 observed, and in tliar cose t/ien were .'•'•. Number of internal longitudinal bars on fold varies from >'> to !t, gem-rally 8 or 9. HERDMAX, p. 330. In considering tinsignificance of these cases with reference to :lnorigin of Species it is to be remembered that the characters of •In- branchial sac, tinsizes of the transverse vessels, shape of meslie-. and tli.- number of M iumata they contain are held to be of •In- first importance for the dassitjeation of Ascidians ; but HERD- MAN finds that while tin-vanhighly characteristic in some species rhrv are not 80 in others'.

*!:}!>. Myxine glutinosa. In this i^-nus thenare normally six pairs .,)' branchial pouches. I am indebted to Professor \Veldon for an account of a specimen dissected by him in which there were pouches Were di-tinet and sepai-ate, each having a separate opening from the oesophagus and a separate aortic arch supplying it. On the right side the sixth and seventh pouches were practically FIG. 27. Myxine rjltttinosa • specimen having seven pairs of branchial sacs. Diagram shewing branchial sacs, heart and aortic arches from the dorsal surface. On the right side the sixth and seventh branchial sacs were partially confluent.