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

There are besides, as in Insects, a considerable number of cases in which the nature of the parts is not clear, though the majority of such cases are not examples of extra parts, but artnormal appendages mutilated or deformed. One specimen (No. 821) is the only case known to me in which two pairs of supernumerary parts arise from one appendage. Another (No. 827) is unique in the fact that according to the description three separate appendages are repeated upon a single appendage. It is not clear that this is in any strict sense an instance of Secondary Symmetry, but for convenience it is taken in this chapter.

Of tinwhole numliiT, two affect antennae, four are in uonchelate ambulatory l'-gs, one is in a chelate ambulatory leg and the real being the great majority, are all in chelae. With reference to these extra parts several false views have from time to time beeE held. For example, in some of the ( •oiniiioiit --t r.i>cs there is an extra pair of dactylopodites, or of indices, curving towards each other. The extra parts may then greatly n--<'inble the dactylopodite (or "pollex") and index of ;i normal chela, and many authors have not unnaturally supposed that the extra parts were actually an extra pair of forceps re- |n-ating tlmse of the normal chela. This may easily be shewn to be an error, from the fact that it is often possible by some slight struetural difference between the pollex and the index to detect that both extra parts are either both pollices or both indices.

But the fullest disproof of this supposition is found in the fact that the great majority of the phenomena will be readily seen tn conform to the principles enuntiated for Secondary Symmetries iu Insects (p. 479). A good many authors from the time of ROSEL VON RosENHOF1 • in wards have said that these cases are a result of injury, or of regeneration after injury. For this belief I know no ground. It should be remembered as an additional difficulty in the way of this belief, that when the limb of a Crab or Lobster is injured it is usually thrown off bodily, while the extra parts most often spring from the periphery of the chela. But since, according to HEINEKEN", such mutilated parts are sometimes retained, this muM not lie insisted on.

In the IMM of an ambulatory leg the surfaces may be named as in an insect (\\ithout any suggestion that these names denote true homolugies between the surfaces so named). In describing chelae I propose to use the following arbitrary terms. The border upon which the dactylopodite articulates is the pollex-border, the i.|ip.i>ite border being the i mles-border. It should be noted that in the Crab the pollex-border is superior, but in a Lobster' itis

*S(),s. Palinurus vulgarise left penultimate ambulatory leg bore two supernumerary legs (Fig. IM)). ( 'oxopodite of great width. The banpodite had three articular surfaces as shewn in Figure 180, 1 It is \\orlh untiring that in tinrh, la of u Scorpion though a close copy of that of a Decapod, tin1 an-aiignm-nt i> reversed, the articulated pincer being external. each bearing a complete leg. When seen by me the leg marked L' was lost. I could not quite satisfy myself as to which of the three was the normal, but it was clear that R' was in form a right leg and that the other two were lefts. If the leg L' is the normal, it has been pushed out of place by a pair of extra legs in Position DAA, but if R' and L' be the extra legs, then the most anterior leg is the normal and has been pushed out of place by a pair in Position VPP. For an opportunity of examining this specimen, I am obliged to the courtesy of Prof. A. MILXE EDWARDS. Originally described and figured by LEGER, M., Ann. Sci. Nut., Zool, 1886, S. vii. I, p. Ill, PI. G.

*§09. Lithodes arctica : 2nd leg on right side has terminal joint as shewn in Fig. 181, II. If R be the normal then R' and L' are a pair in Position V, but if R' be the normal then A' and L' area pair in Position D. Attention called to the great diminution in size of all three terminations as compared with the normal (Fig. 181, I). Original description, HERKLOTS, J. A., Bijdr. tot </. h. Genootsch. Nat. Artis May., 1852, iv. p. 37, PL; repeated Arch, uteri., 1870, v. p. 410, PI. xi.

8H. Carcinus maenas : 2nd amb. leg as in Fig. 181, III. A pair of compounded extra pointin Portion D. Duxs, Proc. I!. l'l<u*. Soc. I i'.. 1^1. I. Lit]i<iili->i n/'i-tii-d, normal terminal joint of ambulatory leg. II. S.-roiid lij-'lit lc_' dt' Nu. 809. (Both after Herklots.) III. Carcinnx mcenas, No. sll, M cond uniliulatory leg. (After Duns.) P, normal terminal point. P\ /'". extra terminal points in Position D. Eriphia spinifrons $ : specimen of unusually large size, normal but for It-it i-hi.-la shewn in Fig. 182, I and II1. The '•ln'l.-i bore normal left dactylopodite, LD, and index, LI; also, upon pollex-border the structures shewn. These consisted of two dactylopodites, II' 1). I.' I), working opposite each other on a compounded double index, /»''/, L'l, which had two toothed borders, one for eaeh of them. This is therefore a pair of chelae repea i • •• I in Position l> [if indeed the dactylopodite mark the dorsal surface]. Taken from 1 1 I:I;K LOTS, Arch. mVr/., 1S70., v. p. 412, PL XI.

1 In rmnirxioii \\itli this case UK UK LOTS states that the rt. chela in the normal ill.- larger ;nid othrrwisr ilittVrs fnun theleft (1 in 8 being reversed in this rrsprrt). It ilors not si cm from the tiguri1 that there was such differentiation ih. f\ira piiir, luit in future cases this point should be looked for. 813. Astacus fluviatilis : about 3 years old according to Sou- BEIRAN'S (Comp. Rend. 1865, LX. p. 1249) account. Right chela apparently deformed by injury or disease. Left chela had all normal

FIG. 182. I and II. Eriphia spinifrons, No. 812. I. A view of the left chela. II. An enlarged view of the extra parts from the other side. LD, LI, normal left dactylopodite and index. R'D, L'D, right and left extra dactylopodites. RI, L'l, right and left extra indices not separated from each other. (After Herklots.) III. Cheliped of Homants americanus, No. 814. (After Faxon.) D, I. normal dactylopodite and index. D', D", extra dactylopodites. I', I", perhaps an indication of double extra indices.

IV. Astacus ttuciatilis, No. 813, left chela. L, normal left dactylopodite. R'D, L'D, right and left extra dactylopodites. L'l+R'I, left and right extra indices not separated from each other. (After Maggi.) parts and in addition the structure shewn in Fig. 182, IV upon the pollex-border of propodite. Here was a boss, separated by a groove. It was observed that the structure was that of a rt. and 1. dactylopodite working upon a double index [as in last case]. Structure of muscles, fully described, was also in agreement with the view that the extra parts were a complementary

814. Homarus americanus : small cheliped as shewn in Fig. 1S2, 111. Il bears ..... -mal dactylopodite (D) and index (7), but this I .a i-i is bent almost at rt. angles. From the outer angle arise tinpartshewn. Apparently D' and D" are a complementary pair iif extra dactylopodites in Position D. The piece I' + I" is imt described; from the figure it seems possible that it may represent parts ,,f the indices proper to D' and D". Case given by FAXON, Harv. Bull., 1880—1, vin. p. 261, PI. II. tig. 2.

Cancer pagurus : right chela as shewn in Fig. 183. This is a case of some conijilcxit v. The tigure will best make it clear. The daetylopodite l> is .single and so also is the index P. D is a double dactylopoditc. and ]" having teeth on two sides may be judged to be a double index. But if D' and P are the normal chela they each stand opposite Fi<;. IHH. C,ui,;-i- ixiiniriix. No. 815. Rfcht cliela seen from the apex, and from the outside. The lettering is aiTan.u'e.l on the hypothesis that D' is the normal ila<-tylo|ic>.lite, /' tlf normal index. /*; the double extra daetylopodite, /'', siiiiill iloiilil.' extra indrx. (From /'/•«.•. Zool. Soc.)

the pincers to which they do not belong. Nevertheless I see no other inter). relation possible. (This ease is curiously like that of the tarsal claws Ln l!hl\«i,-n,ii'K No. 7*11.) Specimen incorrectly described by myself, /'. /.. S., L890, p. 581, iig. 2. C. si7 Uca una : a chela ha\ing complex repetition of parts somewhat (l>) Tit'o e.iii'ii dactylopodites arising from normal dactylopodite. *<Sl,s. T<> this and the next division belong the great majority of

cases of repetition of parts in Crustacea. Including examples recorded by various authors and specimens in different Museums there are nearly fifty cases of this class known to me. FIG. 184. Three cases of two extra dactylopodites arising from a normal dactylopodite. I. Left chela of Carcinus manas in Brit. Mus. II. Left chela of C. mcenas after LUCAS, Ann. Soc. ent. France, S. 2, n. p. 42, PI. I. jig. 2. III. Eight chela of Homarus, after VAN BENEDEN, Bull. Ac. Belg., S. 2, xvn. p. 371.

The various simple forms taken are illustrated by the eight case> shewn in Fig<. 1S4, is:, and 186. It will be seen that when such extra processes arise on the toothed border of the dactylopodite they turn th.-ir * mouth borders to each other, but when FKI. 186. llonianix nnu'ricanus. Three chela} whose dactylopodites bear double extra dactylopoditos. I. A left. II. A left. III. A right. Ii, normal right. L, normal IH't. A1', extra right. L', extra left. (From Faxon.)

they arise on the smooth border they turn their toothed borders to each other, thus fulfilling the conditions of the Scheme given at p. 4S1. Though from the close agreement between the three prongs in some of the specimens it is not always possible to tell the normal dactylopodite with certainty, it will be seen that in these the rules hold whichever of the two possible prongs be supposed to be the normal. 819. Astacus leptodactylus : left chela has dactylopodite as -hewn in Fig 1ST, II. Presumably D is the normal pushed out of place, and D' and D" are the two extra dactylopodites. They are so placed that none meets the index. KAROLI, J., Term. FH:i't,-l.; 1ST 7, I. p. •>:{, PI. II.

FIG. 187. I. Cancer pagurus, No. 820, right chela. Specimen in Coll. Surg. Mus. II. Astacus leptodactyhis, left chela, after Karoli. 820. Cancer pagurus : somewhat similar case in rt. chela (Fig. 187, I) ; but here the normal, R, stands in its normal place. In Coll. Surg. Mus. *821. Homarus americanus : dactylopodite only of right chela preserved. It is bent sharply downwards, out of the plane of the " hand," and bears upon its upper surface two pairs of blunt, toothed processes [probably being rudiments of two pairs of extra dactylopodites]. FAXON, I.e., p. 261, PI. II. fig. 1.

822. Homarus americanus : dactylopodite (a) bent upwards and outwards, crossing index without meeting it (Fig. 188). From the smooth border of dactylopodite arise two toothed processes FIG. 188. Homarus americanus, No. 822, left chela, a, normal point of dactylopodite. b, c, extra points. (After Faxon.) (b and c) curving towards index. [I take it that this is something like the cases of Position A in Insects (p. 481) but from the original figure the relations cannot be quite decided.] FAXON, I.e., p. 260, PI. I. fig. 15.

(c) Tiro c.rtrn i IK I ices arising from a normal index. *823. This again is a fairly common form, though much less frequent FIG. 189. I. Eight chela of Homarus americanus. R', L', right and left extra indices not separated from each other. (After Faxon.) II. llomaritx I'ulguris, ri^ht chela in Brit. Mus. III. //. ruli/nria, right chela bearing extra double index. li ;md L', not separated. (After LUCAS, I.e.) Fm. 1HO. I. Left chela of Cun-iun* itio'ini*, indices only shewn, rf, place of articulation of dactylopodite. In Coll. Kurg. Mus. II. A similar case in lli'iii'irna iiiiii'rii-iiniis, nfti'r Faxon. /,, normal left index, li', L', extra right and left indices.

than the last. The cases known to me amount to about ten or fifteen. Seven cases are illustrated in Figs. 189, 190, and 191. FIG. 191. Two cases of extra indices in Cancer pagurus. I. In Coll. Surg. Mus. II. After le Senechal. R, normal right index. L, normal left. R', L', extra rights and lefts. (d) Simple processes, probably being rudimentary extra pairs of indices or of dactylopodites. *824. Many such are described, but of few can anything be said with confidence. A comparatively simple case is shewn in Fig. 192, where there is a decided suggestion that the process L' + R' is morphologically a pair of indices that have not separated from each other but stand compounded by their toothed borders. On comparing this case with for instance, Fig. 191, II, it will be seen that the two conditions might readily pass into each other in the way so often seen in Insects.

Other cases of a more doubtful character are shewn in Fig. 193. Though in each the nature of the extra part is obscure, it is probable that they are all rudimentary states of the repetitions described. The alternative view that they are single repetitions certainly can- Fio. 192. Left chela of Portunot be applied to all, for in many the UJIS P"bc>' {Jom ^ SENECHAL, Hull i-xtra process, though in the plane of the L> normal illdex L, + /r%pah. index and dactylopodite, is similar on of extra indices in Position V.

I'm. 193. I. Eight chela of C. pagurus in Coll. Surg. Mus. /,'. ri^-lit index. II. Similar s]>. rimen whose dactylopodite bears x, a supernumerary process. In Coll. Surg. Mus. III. Attacus fluviatilis, left chela bearing .r, a supernumerary process. /,'/, III), right index and dactylopodite. (After LUCAS.) both its faces in this plane. There is however no doubt that the tli-tinctidii between these cases and true duplicity is hard to trace and jiuxMbly enough it is not really absolute.

A.-* each case differs from the others I give a list of those not in private collections1. The ? indicates that the case perhaps approaches the condition of true duplicity . cyliiiili-ir;il instead of fiattoned ; peripherally divides into two ]»arts each bearing an articulated appendage as shewn in Fig. li»4 [The appendage R i> a normal chela. What is R -t- L' ? FAXON, carefully describing the case, thinks that R + L' is a rudimentar\ and reversed ropy of 7i', and that the ease is one of duplicity, lint from the particulars given, and especially from the circumstance that the carpopodite was "much more spiny' than tin

-mal. I think it likely that A" + // is morphologically a double structure formed of a jtnir of carpopodites compounded together. 1 \Vitli these may pnli.1]., lie mentioned the following: Apus cancriformis. having upon the K>th foot a second -mall llabelhim shaped like the normal flabellum. The In act was greatly reduced in size. LANKKSTKK, E. E., y.J.M.S., 18S1, xxi. p. 350, PI. xx. tig. 12. [In explanation of Plate the abnormal foot is called the 30th.]

Without having seen the specimen it is impossible to say much, but the parts should be examined with a view to this possibility. I conceive that the large spine marked by Faxon sp' stands on the morphologically middle line between the two extra halfmeropodites.] FAXON, Harv. Bull. vin. p. 2G2, PL II. fig. 6. *827. Astacus fluviatilis $ : large adult. i. Abdomen wide in comparison with slender chelse: otherwise normal except left chela. This was formed as in Fig. 195. All normal except carpopodite, from which arose a fixed piece seeming to be an extra misshapen carpopodite, bearing three extra chelce, L', R and x. [R1 and L' are a clear pair of images L' being right and left respectively. But between R and the normal L there is the third extra chela x. As to the nature of this nothing can be said. Whether it is a left or a right cannot be told from fig. So far as I know,

FIG. 195. Astacus fl this case is unique. Full description 827, left chela. L, the normal. J!\ *828. Palinurus vulgaris : right antenna bore three complete filaments. So tar as last spiny joint (merocerite) normal. Of this joint the peripheral portion much enlarged, presenting two articulations The most posterior bore a normal rarpoeerite and filament (Fig. 1!»C, I). The anterior articulation bore a double carpocerite with two ti laments (II and III). As author points out, II is structurally a /<;// antenna. [By the kindnes- ,,f M. Alphonse Milne Kd wards I have been allowed to examine thU specimen. I am not -ure that I sue, -ceded in correctly determining the surfaces of the extra antennae, for the basal parts were not very fully formed ; but according to my determination

from those of any of the Schematic antenna of Palinurus ruiaaris, No. 828. positions, for while the position I, the normal. II, extra left. Ill, extra of origin is VVA the two extra right. (After Leger.) antenna' stand very nearly in the Position DA.] L£GER, Ann. svi. nat., Znn/.. issi;', S. 7, i. p. 1UO, PI. (i. *<S29. Astacus fluviatilis : exopodite of left antenna! Fig. l!)7 )bear- t wo supernumerary points, A'' and //, which seem to have been insc-rieil upon the internal border of the normal exopodite. STAMATI, G., Ball. Soc.

As has been stated, there are many cases, recorded or preserved, in which the nature of the parts cannot lie made out. The majority of these are, I believe, injured or deformed limbs, ami not cases of repetition of parts. \e\ert liel<\ss of the latter class there are un- FIG. 1U7. Axtucu* Jlnriatilix having extra points to «'X(i]ioditc of left antenna. I?, dOUbtedly some amorphous cases, though they normal right. L normal left, are far less common than regular ones, even A", //, extra right and left. as normal st nictures arc more common in their (After Stamati.) regular shapes than in a deformed state. I mention the following as bein.i:. I think, i he earliest record of abnormalities of this class.

Homarus : left chela having irregular process on inner border of dactylopodite, and two irregular processes on inner border of index. [No description.] I'.KKMI \ i;nrs \ JJKKXIZ, Miscell. Curios., Jena, n. 1671, p. 17.">. Obs. 01. /'/. THAT there should be such a thing as a limb double in the sense in which the following are double, has always seemed to me most strange. We know that a segment of an Annelid, or a vertebra, may be on one side of the body divided to form two segments or two vertebra (as in No. 88 or No. 7) while on the other side of the middle line the segment is single. This is in keeping with all that we know of Division of parts in Linear Series. So might we suppose that a parapodium, or a rib, or perhaps a litnb-bud might divide into two ; but the two halfsegments or half-vertebrse are in Succession to each other, and are not complementary images of each other as these doublelimbs are.

That a parapodium may divide into two Successive parapodia is possible enough, though, apart from division of the segment bearing it, I know no clear case. But it may be stated at once that in Arthropods and Vertebrates such a phenomenon as the representation of one of the appendages by two identical appendages standing in Succession is unknown. No right arm is ever succeeded on the same side of the body by another arm properly formed as a right, and no Crustacean has two right legs in Succession, where one should be. The only cases at all approaching this state are those of Macacus No. 504 (q. v.), a case that must be interpreted with great hesitation ; and of the Frogs described by CAVANNA and by KINGSLEY, also doubtful cases (see Chapter xxni).