CHAP, xxiv.] AXIAL DUPLICITY : INVERTEBRATES. 563 



See also, GEOFFROY ST. HILAIRE, Hist, des Anom., ed. 1838, n. p. 

 197 ; DUMERIL et BIBRON, Erpet. generate, 1884, vi. p. 209. 



866. Duplicity of the head is less common in Lizards, but several 

 examples are known. See GEOFFROY ST. HILAIRE, I. c., p. 195 ; Cosmos, 

 Paris, 1869, S. 3, v. p. 136, <fec. 



*867. In Chelonia also are several such instances. See EDWARDS, Nat. 

 Hist, of Birds, d-c., Pt. iv. 1751, p. 206; MITCHILL, I. c. ; BARBOUR, 

 E. H., Amer. Jour, of Sci., 1888, S. 3, xxxvi. p. 227, PI. v. The last is a 

 particularly interesting case from the circumstance that the behaviour 

 during life was observed to some extent, though only a popular account 

 is given. The two heads seemed to act independently, and it is said 

 that there was no concerted action between the feet of the two sides. 

 BARBOUR'S figures are reproduced in Fig. 207. 



In fish-hatching establishments double monstrosity is of frequent 

 occurrence among young Salmon and Trout. A two-headed embryo of 

 a Shark is preserved in Coll. Surg. Mus. (Terat. Cat. 1872, No. 22). 



The following cases relate to invertebrates. 



Chaetopoda. Duplicity in this Class has been often seen, but 

 that any of the cases are truly congenital cannot be stated. There is 

 evidence that in many Annelids regeneration 1 both of head or tail may 

 freely occur, and it is quite possible that the second head or second tail 

 may have grown out from an injured place, though of this there is no 

 actual proof. In cases of posterior bifurcation each tail generally 

 contains all the parts proper to the normal, but in No. 871 one of the 

 tails was without the terminal cirri usual in the species. So far as can 

 be gathered from the evidence it does not appear that the two con- 

 tinuations of the body have always the same number of segments, 

 which might perhaps be expected were both the result of a natural 

 division of the developing body. On the other hand, they do seem 

 generally to have a nearly equal development, and are almost always 

 (in cases of double tails, at least) fairly equal in length, which would 

 not be anticipated if one only were a new growth. Moreover, if the 

 double tail is in some way due to regeneration one would expect to 

 find such duplicity in its minor conditions much more commonly. 



Into the details of the structure it is not now proposed to enter, 

 and indeed of most of the cases there is little to be told. The evidence 

 is mentioned here simply in further proof of the power of these indiv- 

 iduals, thus greatly departing from the normal of their species, to 

 maintain themselves with no apparent difficulty. It will be noticed that 

 the species concerned are most various, and include not only Errantia, 

 but two cases also in Serpulidfe. 



The literature of the subject was collected by CoLLiN 2 , and a list of 

 the references was independently collected and published with abstracts 

 by ANDREWS 3 . This list, with a few additions, was republished by 

 FRIEND 4 . Though many of the accounts are imperfect they are referred 



1 The evidence on this point does not come within the scope of this work. 

 References to it may be obtained from ANDREWS, ZEPPELIN, &c. (v. infra). 



2 COLLIN, A., Naturw. Wocheiis., 1891, No. 12, p. 113. 



3 ANDREWS, E. A., Amer. Nat., 1892, xxvi. p. 729. 

 - 1 FRIEND, H., Nature, 1893 (1), p. 397. 



362 



