^11 PHYLUM MOLLUSCA. 733 



contains distinct cerebral and pleural, besides pedal, visceral, abdo- 

 minal, and buccal ganglia. The sexes are sometimes separate, 

 sometimes united. The larva passes through trochophore and 

 veliger stages. 



Sub-Class I.— Streptoneura. 



Gastropoda in which the visceral connectives are in most cases 

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



Order 1. — Aspidobranchia. 



Streptoneura with the nervous system but little concentrated : 

 the pedal ganglia are produced into long cords with the anterior 

 ends of which the pleural ganglia are fused ; the cerebral ganglia 

 wide apart; the osphradium little developed. There is nearly 

 always a single ctenidium or a pair, plume-like and free distally. 

 The auricles and the nephridia are usually paired. 



Siib-Order 1. — Docoglossa. 



Aspidobranchia in which the pleural ganglia are not connected 

 with the opposite visceral connective. The eye is in the form of 

 an open pit, without lens. There are two osphradia, a single jaw, 

 and no operculum. The visceral mass is conical. 



This section includes the Limpets {Patellidce). 



Siti-Order 2. — -EMpidoglossa. 



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 {Haliotidm), TrocJms, 

 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. 



Stih-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. 



