xn PHYLUM MOLLUSCA 715 



Sub-Order 2. Nudibranchia. 



Opisthobranchs which are devoid of a shell in the adult condition, 

 and have no true ctenidia or osphradia, respiration being carried 

 on by means of secondary branchiae usually arranged in a circlet 

 around the anus, or in rows on the dorsal surface, or laterally under 

 the edge of the mantle. 



This sub-order includes Doris, Eolis, Tethys, and other shell-less 

 forms. 



OEDEE 2. PULMONATA. 



Euthyneura devoid of ctenidia, respiration being carried on 

 through the walls of the mantle-cavity, which has a narrow con- 

 tractile aperture. 



This sub-order includes the Land-Snails and Slugs. 



Systematic Position of the Example. 



Triton rubicundus is one of several species of the genus Triton, 

 which is the only member of the family Tritonidce belonging to 

 the sub-order Platypoda. The family Tritonidae differs from the 

 other families of the sub-order in the possession of a proboscis, of 

 a well-developed, but not greatly elongated, siphon, and of a short 

 foot, 



3. GENEEAL OEGANISATION. 



External Features, Symmetry, &c. Few T Gastropods make 

 an approach towards even superficial symmetry, and in cases in 

 which there is a near approximation towards such a state of 

 things, it seems clear, from the results of the study of develop- 

 ment and of a comparison with allied forms, that the symmetry 

 presented is not primitive, but has been secondarily acquired 

 such symmetrical forms having been derived from unsymmetrical 

 ancestors. 



The departure from symmetry is most marked in the majority 

 of the Streptoneura. It may be said to be due to the develop- 

 ment of a protective shell composed of one piece and extensive 

 enough to be capable of enclosing all the soft parts ; and to 

 the extension of the foot on the ventral side as an elongated 

 muscular creeping organ. The development of the shell rendered 

 necessary an arrangement of the parts whereby the mantle-cavity 

 with the anus, the ctenidia, and the excretory apertures should 

 come to be situated in the neighbourhood of the opening of the 

 shell, i.e., towards the head-end of the animal. The mantle- 

 cavity and associated parts (pallial complex, as the whole is termed) 

 had, therefore, to be shifted forward from its primitive posterior 

 position, and this was probably effected by arrest or retardation of 

 growth on one side and active extension on the other.. In the 

 majority of cases it is the right side the growth of which becomes 



VOL. I 7 7* 



