164 GASTROPODA. 



opens out near the anus as an anal gland. In Ercolania, this gland is un- 

 paired and strongly pigmented. A glandular structure described by RHO in 

 Chromodoris is said also to open near the anus. This involuntarily recalls the 

 rudiment of the kidney of the adult, a view which has recently been adopted 

 by MAZZARELLI (No. 74 and No. XV.). This author derived similar structures 

 from the mesoderm. One organ especially which, curiously enough, was 

 assumed to be an " anal eye," excited attention. This lies in various Opistho- 

 branch larvae (in Aplysia, Philine, Pleurobranchus, Doris, Aeolis, LACAZE- 

 DUTHIERS and PRUVOT, No. 60) on the ventral side, near the anus ; it is 

 strongly pigmented and is no doubt identical with the glandular structures 

 above mentioned. According to MAZZARELLI, as already mentioned, it is 

 derived from the mesoderm, but LACAZE-DUTHIERS and PRUVOT, who in- 

 vestigated the origin of this hypothetical larval eye more closely, traced it 

 back to the ectoderm. This was also the result of the ontogeiietic researches 

 of HEYMONS as to the origin of this structure, and it cannot therefore be 

 regarded as a nephridium, but must rather be compared with those excretory 

 organs which, like the sub-velar cells described in the Prosobranchia, are 

 yielded by the ectoderm (p. 129). The excretory character of the organ seems 

 indisputable, but no decision as to its homology can be arrived at until its 

 development and future fate in the different forms of Opisthobranchs are 

 better known. 



Among the sensory organs of the larva, the large otocysts at the 

 base of the foot deserve special mention. As in other pelagic larvae, 

 strong cilia appear at the centre of the velar area in various forms 

 (Fiona, Polyeem, Elysia, Philine, HADDON, No. 40). In the Aeolidae, 

 the end of the foot carries a few long stiff cilia. Eyes are found on the 

 velar area in those cases at any rate in which tentacles also appear 

 there as rudiments, but are altogether wanting in many larval forms. 



The greatly modified forms found among the Opisthobranchia, 

 such as the genera Limapontia and Pliyllirhoe, like the more primitive 

 forms, have larvae with bilobed velum and shell provided with an 

 operculum ( ABLER and HANCOCK, No. 2; SCHNEIDER, No. 112). 



Our knowledge of the transformation of the larva into the adult 

 rests principally upon the statements of MAX SCHULTZE and NORD- 

 MANN made with regard to Tergipes Edwardsii and T. lacinnlatus 

 (Nos. 80 and 113). 



The larva of Tergipes Edwardsii, when still provided with a shell, 

 already seems to have lengthened somewhat. The two velar lobes 

 are unusually large and oval. On the velar area are situated a pair 

 of tentacles and, at the base of these, the eyes. The larvae probably 

 swim about for some time at this stage. The mantle then with- 

 draws from the shell and comes into closer contact with the body. 

 The way is thus prepared for the casting of the shell which takes 

 place while the velum is still fully developed. We thus have a 



