MOLLUSOA OF PLYMOUTH. 451 



celleut figures of this condition are to be seen in Alder and Han- 

 cock's Monograph. I find, however, that in very large individuals 

 continuous growth may entirely obliterate all trace of the fusion 

 which has taken place, so that a continuous, even, and circular fold is 

 formed around the back {iiothseum) of the animal, exactly as in 

 Archidoris, Lamellidoris, &c. PI. XXVII, fig. 6, represents this con- 

 dition in a preserved specimen measuring -^ inch in length and | inch 

 in breadth. 



The process of growth thus described as taking place in the indi- 

 vidual Goniodoris nodosa throws considerable light upon the question 

 of the origin of the circular fold of Archidoris. It is many years 

 since Huxley"^ suggested its homology (in part) with the paired 

 '' epipodia " of Aplysia ; but although the suggestion has met with 

 approval" little direct evidence has been collected in support of the 

 view. The ontogeny of Goniodoris nodosa, however, shows conclu- 

 sively that the circular fold has been arrived at by a process of posterior 

 fusion of a pair of lateral folds ; for the anterior union is clearly 

 also secondary : the primitively discontinuous condition is persistent 

 in Polycera lessonii. Therefore the origin of the circular fold of 

 Archidoris and its allies from primitively paired lateral folds can no 

 longer be considered as doubtful; and the existence of transition 

 forms like the Lomanotidse and Ascoglossa renders the homology of 

 these paired folds with the " epipodia ^' (or better, " pleuropodia ") 

 of Aplysia almost certain. 



In some way, perhaps, related to the fusion of the pleuropodia 

 posteriorly is a curious transparent spot shown in my figure of the 

 young Goniodoris nodosa, situated between the anus and the termina- 

 tions of the folds. This was mistaken by Johnston (Ann. Nat. 

 Hist., vol. i, p. 55) for a pore, but there is no perforation. Alder 

 and Hancock rightly corrected this mistake, but fell into error in 

 adopting Allman's explanation of it. Allman informed Mr. Thompson 

 (Ann. Nat. Hist., vol. v, p. 88) that the pore-like appearance was 

 " merely formed by the partial apposition of the edges of a slit 

 existing in the posterior margin of the mantle, and which approxi- 

 mation is dependent on the will of the animal ;" and Alder and 

 Hancock followed him in stating that the spot was merel}'^ " caused 

 by a deep indenture of the cloak." This is not the case, for the 

 spot in question is simply an oval area from which the minute 



^ Huxley, Morphology of the Cephalous Mollusca, Phil. Trans., 1852. 



2 Cf. Lankester, Mollusca, Encycl. Brit., 9th ed., vol. xvi ; Fischer, Manuel de Con- 

 chyliologie, 1887, p. 518 (Fischer's view is that the nothajum of Doris represents at the 

 same time both the "epipodial" lobes of Elysia or Aplysia fused ia the median line and 

 the cephalic disc of Philine) ; Hcrdman and Clubb, Third Report on the Nudihranchiata, 

 1. c, p. 147 ; also my Report on the Nudihranchiata, 1. c, p. 181. 



