734 ZOOLOGY . sect. 



Sub-Order 2. — Heteropoda. 



Pelagic Pectinibranchia with the foot laterally compressed and 

 bearing, at least in the male, a ventral sucker. The visceral sac 

 and mantle form only a small part of the mass of the body. Jaws 

 are absent. 



Sub-Class II.— Euthyneura. 



Gastropoda in which the visceral connectives are not twisted 

 into a figure of 8, and in which the sexes are united. 



Order 1. — Opisthobranchia. 



Marine Euthyneura with aquatic respiration, the auricle of the 

 heart usually posterior to the ventricle. The mantle-cavity, when 

 present, opens by a wide aperture. 



Sub- Order 1 . — Tectibranchia. 



Opisthobranchs provided in nearly all cases with a mantle and 

 a shell, nearly always with a true ctenidium, and an osphradium. 



This section comprises the Aflysiidw, or Sea-hares, and several 

 other families, including certain pelagic Gastropoda, some shell- 

 bearing, some shell-less, formerly regarded as constituting a 

 distinct class — the Pteropoda. 



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 tinder 

 the edge of the mantle. 



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

 forms. 



Order 2. — Pulmonata. 



Euthyneura devoid of ctenidia, respiration being carried on 

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

 contractile aperture. 



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



Systematic Position of the Example. 



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

 which is the only member of the family Tritonidw, belonging to 

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



