xii PHYLUM MOLLUSCA 683" 



taking place through the wall of the mantle-cavity itself. A dis- 

 tinct head bearing eyes and tentacles is present in the majority. 

 The foot is situated behind the head, and usually has an extensive 

 flattened ventral surface. The buccal cavity contains an ud<>nto- 

 phore. The nephridium is usually single. The nervous system 

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

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

 sometimes united. The larva passes through trochosphere and 

 veliger stages. 



Sub-Class I. Streptoneura. 



Gastropoda in which the visceral commissures are 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 produced into long cords with the anterior ends 

 of which the pleural ganglia are fused, the cerebral ganglia wide 

 apart, the osphradium little developed. The ctenidia, nearly 

 always present, are plume-like and free distally. The auricles, 

 and the nephriclia are paired. 



Fub-CrJer 1 . Docoylossa. 



Aspidobranchia in which the pleural ganglia are not connested 

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

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

 and no operculuin. The visceral mass is conical. 



This section includes the Limpets (Patdlidm). 



4 



Sub-Order 2. PJiipidoglossa. 



Aspidobranchia in which each pleural ganglion is connected 

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

 and contains a lens. There is nearly always a single osphradium. 

 (the primitive left), a pair of jaws, and two auricles in the heart. 



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

 T'lirlo, and others. 



ORDER 2. PECTINIBRANCHIA. 



Streptoneura with a somewhat concentrated nervous system. 

 There is a single osphradium which is often pectinate. The 

 primarily left ctenidium and nephridium are alone developed. 

 The heart has a single auricle. The ctenidium consists of a 

 stem with a single row of lamelloe, attached throughout its length 

 to the wall of the mantle-cavity. 



