714 ZOOLOGY SECT. 



Sub-Order 2. Rhipidoglossa. 



Aspidobranchia in which each pleural ganglion is connected 

 with the opposite visceral connective. The eye is a closed sac 

 and contains a lens. There are nearly always a single osphradium, 

 a pair of jaws, and two auricles. 



This sub-order includes the Ear-shells (Haliotidce), Trochus, 

 Turbo, and others. 



ORDER 2. PECTINIBRANCHIA. 



Streptoneura with a somewhat concentrated nervous system. 

 There is a single osphradium which is often pectinate. The 

 primarily right ctenidium is alone developed. The heart has a 

 single auricle. The ctenidium consists of a stem with a single row 

 of lamellae, attached throughout its length to the wall of the 

 mantle-cavity. 



Sub-Order 1. Platypoda. 



Pectinibranchia with the foot flattened ventrally, at least in 

 front. Jaws are nearly always present. 



This sub-order includes the Cowries, the Vermetes, the Tritons, 

 the Whelks, the Cones, and a number of other groups. 



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 whicn 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 Aplysiidce, 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. 



