MOLLUSCS AND BRACHIOPODS. 441 



Thus, in Patella the eye is scarcely more than an invagination or 

 depression in the integument, which is lined with pigment and 

 retinal cells. The next upward stage occurs in Trochus^ where 

 the depression becomes deeper and bladder-shaped^ and is filled 

 with a gelatinous or crystalline mass, but still is open at the top, 

 and therefore permits the eye to be bathed in water. Then, as in 

 Turbo, the bladder becomes closed by a thin epithelial layer, 

 which finally, as in some Murex, become much thicker ; while the 

 ' eye-ball ' encloses a lens (Fig. 2), which probably corresponds 

 with the ' vitreous humour ' of other types. 



In Chapter VIII, is described The Digestive Organs, Jaw, and 

 Radula, and Excretory Organs. As the mouth-organs are always 

 specially interesting to microscopists, we make a few short extracts. 



The mouth is generally, as in the common snail and periwinkle, 

 placed on the lower part of the head, and may be either a mere 

 aperture, circular or semi-circular, in the head-mass, or, as is more 

 usual, may be carried on a blunt snout, which is capable of varying 

 degrees of protrusion. From the retractile snout has doubtless 

 been derived the long proboscis, which is so prominent a feature 

 of many genera. ... As a rule, Mollusca provided with a 

 proboscis are carnivorous, while those whose mouth is on the 

 surface of the head are vegetable feeders ; but this rule is by no 

 means invariable. 



The Pharynx, J^i^s, and Radula. — Immediately behind the 

 lips the mouth opens into a muscular throat, pharynx, or buccal 

 mass. The pharynx of the Glossophora — i.e., of the Gasteropoda, 

 Scaphopoda, and Cephalopoda — is distinguished from that of the 

 Pelecypoda by the possession of two very characteristic organs for 

 the rasping or trituration of food before it reaches the oesophagus 

 or stomach. There are {a) the Jaw or jaivs, and (b) the radula/"' 

 odontophore, or lingual ribbon. The jaws bite the food, the radula 

 tears it up small before it passes into the stomach to undergo 

 digestion. The jaws are not set with teeth like our own ; roughly 

 speaking, the best idea of the relations of the molluscan jaw and 

 radula may be obtained by imagining our own teeth removed from 

 our jaws and set in parallel rows along a greatly prolonged tongue. 



* Radere, to scrape ; hlo^g, tooth ; tpipuv, to carry. 



International Journal of Microscopy and Natural Science. 

 Third Series. Vol. V. ff 



