ASPIDOBRANCHIATA. 



393 



the left half ventral to it. The pleural ganglia are sometimes 

 joined to the opposite half of the visceral commissure by an anas- 

 tomotic branch of the pallial nerve (dialyneury, e.g., Cassidaria, on 

 left side), or the pallial nerve directly joins the ganglion on the 

 opposite half of the visceral commissure, i.e., the pamal nerve 

 from the right pleural joins 

 the sub-intestinal ganglion, 

 and that from the left 

 pleural joins the supra- 

 intestinal (zygoneury, more 

 common on the right side, 

 e.g., Cassidaria, where the 

 right pleural is directly 

 joined to the sub-intestinal, 

 see above, p. 365, Fig. 286). 

 The males are often pro- 

 vided with a large penis 

 placed on the right anterior 

 side of the body. In the 

 generative organs accessory 

 glands are usually absent. 

 The eggs are surrounded 

 by albumen and laid in 

 capsules, which are fre- 

 quently fixed to foreign 

 objects; sometimes they 

 are attached to a raft of 

 mucous bubbles, to which 

 the animal itself adheres 

 (lantliina). 



FIG. 310.^-Nervous system of Haliotis (diagrammatic 

 after Spengel). Cg cerebral ganglion ; Pg fused 

 pleural and pedal ganglia ; right and 0' left 

 osphradium ; Ag abdominal ganglion ; Pe pedal 

 cord ; S and S' pallial nerves ; Br gills. 



Sub-order 1. Aspidobranchiata (Diotocardia, Scutibranchia). 



Ctenidium Itipedinate or absent, labial commissure present. 



The pleural ganglia are approximated to the pedal, which are 

 prolonged posteriorly into long ganglionic cords (Figs. 283, 310). 

 The cerebral ganglia are widely separate from each other, and the 

 commissure connecting them passes in front of the buccal mass and 

 salivary glands. There is a sub-oesophageal labial commissure with 

 buccal ganglia (Fig. 283). The osphradium is small, and placed on 

 the branchial nerve. The otocyst has numerous small concretions 

 (otoconia). The eye is open (Fig. 290 A), or closed with a very 



