734 ZOOLOGY sect. 



Stib-Ordcr 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. 



Sith-Order 1. — Tedibranchia. 



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 Aplysiidcc, 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. — Nudihranchia. 



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 branchife 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. 



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 Tritonida:, belonging to 

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



