xii PHYLUM MOLLUSCS 733 



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

 ininal, 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. 



Sub- 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 bperculum. The visceral mass is conical. 



This section includes the Limpets (Patellidce). 



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), Trochu^, 

 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. Platypcda. 



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. 



