1894.] ME. WALTER GAESTANG ON COLPODASPIS PtTSILLA. 665 



frequently assumed in captivity in order to creep, after the manner 

 of so many Nudibranchs, along the surface-film, a large glandular 

 mass of an orange colour could be seen through the skin in the 

 anterior part of the posterior prolongation of the mantle, where 

 this organ lay beneath the foot. This glandular mass of an orange 

 colour in all probability represents the "rounded brownish-yellow 

 mass " observed by Sars in a similar position and termed by him 

 the Uver. The anterior edges of the foot, the dorsal and posterior 

 edge of the tentacles, and parts of the ventro-lateral region of the 

 mantle were ciliated. 



The animal consists of a foot, a small tentaculated head, an 

 elevated globose body, and a posterior tail-like pallial appendage. 



The Foot. — Sars states that the foot is well-developed and of 

 about the same length as the mantle ; that in front it is as 

 broad as the mantle, but becomes considerably narrower behind, 

 and terminates in an obtusely rouiided extremity. He further 

 states that its anterior edge is divided in the middle by a deep 

 incision into a pair of lappets with rounded extremities. These 

 statements are perfectly borne out by his figures (pi. xi. figs. 1, 4); 

 but comparison with those supplied by myself shows that a some- 

 what different interpretation must be made of the anterior parts 

 of the foot. The two lappets, which in Sars's figures are shown 

 to be directed forwards, are not really, as he maintains, the divari- 

 cated halves of the anterior part of the foot, but are rather to be 

 regarded as a pair of expansions of the antero-lateral margins of 

 the foot, analogous to the anterior horns of the foot in many 

 Solids, but differing from the latter in their greater size and 

 obtuse extremities (PI. XLIV. fig. 2). Sars's figures also indicate 

 that they are capable of being directed forwards ; but I never 

 observed them in this position myself, and must regard the con- 

 dition represented in my figures as more normal than the former. 

 These antero-lateral processes are so considerable that, in view of 

 the afiinities indicated by other organs of Colpodaspis, I am strongly 

 inclined to regard them as homologous with those pleuropodial * 

 expansions so frequently met with among Opisthobranchiate mol- 

 lusks. This view receives strong support from the fact that in 

 Haminea hydatis of the Mediterranean (which appears to be a 

 different species from the H. hydatis of British naturalists) the 

 pleuropodia, according to Eoide^, are scarcely developed except on 

 the sides of the anterior region of the body. Here — to judge from 

 Eoule's figure — they form elongated obtuse flattened expansions 

 of the foot remarkably like those of Colpodaspis, differing only in 

 their greater size and in their power of retroflexion over the back 

 of the body. 



1 The term pleuropodia was suggested by me in 1890 as a substitute for the 

 undesirable word parapodia as applied to tlie lateral pedal expansions of Opis- 

 tbobranch moUusks, and has been accepted by Bergh and other writers (Journ. 

 M. B. A. i. p. 419). 



^ Eoule, " Eecherches sur les Tectibranclies etc.," Ann. Mus. Hist. Nat. 

 Marseille, ii. 1885, Mem. no. 3, p. 22, fig. 13. 



