8 MOLLUSCA. 



In a work like the present, which is intended to be 

 entirely Zoological, the anatomical and physiological de- 

 tails are purposely omitted. The zoological peculiarities 

 of the adult must now be briefly considered. 



The head in some families is permanently extended and 

 proboscidiform, as in Littorinida and Melaniidee. In the 

 predaceous Gasteropods it is furnished with a muscular 

 trunk, which is often entirely retractile ; but in the phyto- 

 phagous tribes it is supplied with jaws for compressing 

 the food. The head is often inconspicuous, as in many 

 Muricidte ; in some it is long and flat, as in Bursa and 

 Sycotypus ; in some it is produced and cylindrical, as in 

 Cassidulus ; in some broad and flat, as in Bulla and 

 Luniceps. 



Some Mollusks are blind, as the Eolis^ Glaucus, and 

 Doris, and, in the adult state, the Conchifera. The eyes 

 make their first appearance as coloured specks on the 

 upper surface of the neck, as in Apl^siidce, or on the head, 

 as in Bidlidfc. In their most i3erfect form they have 

 iris, pupil, crystalline lens, choroid, vitreous and aqueous 

 humours, and cornea, as in Cywibium, Helix, Stromhus, 

 and the Cephalopods. The eyes are two, symmetrical, 4 

 usually at the sides of the head, sessile or pedunculated. 

 In Haliotis, Nerifa, Turho, Helix, and OncMdium, they 

 are at the end of long peduncles, or ommatopliora. In 

 Muricidm and Cyprwidm they are extended on tubercles, 

 connate with the tentacles ; but in Stromlidw they are | 



free. The vision of Mollusks varies with their habits. 

 In the Bivalves and Tunicaries the eyes are readily 

 dispensed with, the animals being either fixed or nearly 

 motionless. In the Pectens, however, which swim rapidly 

 by the flapping of their valves, the eyes are numerous 

 and pedunculated, and situated at the bases of the pallial 



