CLASSIFICATION OF MOLLUSCS 



471 



er 



Pretre. 



Class I. Gasteropoda 



.Molluscs with a usually well-developed head region with 

 tentacles and odontophore. The foot 

 is usually a flat median sole on which 

 the animal creeps ; it is often divided 

 into pro-, meso-, and meta-podium. 

 Most are unsymmetrical, but there is 

 a primitive bilateral symmetry in 

 Isopleura and a secondary superficial 

 bilateral symmetry in some pelagic 

 forms such as Heteropods . The mantle 

 or covering of the visceral sac usually 

 forms a well-marked fold or flap 



where the visceral sac joins the head fig. 268. Chiton. Aft 



and foot, and thus encloses a mantle 

 cavity. In most cases the shell is a 

 single piece ; in Chitons there are 

 eight pieces ; in many cases the shell 

 is rudimentary or absent. There is 

 usually a trochosphere and veliger 

 larva, except in terrestrial forms. 



Sub-class I. Gasteropoda 

 Isopleura 



The Isopleura are marine Gastero- 

 pods more or less elongated in form, 

 with bilateral symmetry. The sym- 

 metry is not only seen in the form of 

 the body, but in the numerous ctenidia, 

 the paired nephridia, auricles, and 

 genital ducts. The shell consists of . 

 eight pieces. The mouth is anterior ; 

 the anal and nephridial apertures are 

 posterior. The mantle, which bears 

 cuticular spicules, covers at least a 

 great part of the body. The nervous 

 system consists of a cerebral com- 

 missure and two paired longitudinal 

 cords (pedal and visceral), with 



Fig. 269. — Dorsal view of 



nervous system of Acan- 



thochiton. — After Pel- 



seneer. 



I, Upper buccal commissure; 

 2, upper buccal ganglion ; 3, 

 stomatogastric commissure ; 

 4, labial commissure; 5, sub- 

 radular ganglia ; 6, anterior 

 pedal commissure ; 7, pedal 

 nerve with palliopedal con- 

 nections ; 8, supra-rectal 

 pallial commissure ; 9, pallial 

 nerve ; 10, anastomosis of 

 branches of pedal nerves ; 1 1 , 

 stomatogastric ganglia; 12, 

 oesophageal nerves; 13, cere- 

 bral commissure. 



