GASTROPODA PROSOBEANCHIATA. 45 



most Cephalophora (Odontophora) a large muscular mass, the tongue, 

 covered by a hard chitinous plate, the radula, is found upon the floor 

 of the buccal cavity. The heart, on the other hand, more nearly 

 resembles in structure and position that of the Lamellibranchs, in that 

 it consists of two auricles opening into a median ventricle, which lies 

 over the rectum. 



The kidneys are paired, and open right and left in the mantle 

 furrow ; [they also open, as in other Molluscs, into the pericardium]. 

 The Placophora are dioecious. 



Testes and ovaries are simple unpaired glands, which lie im- 

 mediately over the liver and alimentary canal ; their ducts open on 

 each side into the mantle cavity in front of the kidneys. 



The development of the egg begins with an equal segmentation ; 

 subsequently the segments of one-half of the ovum divide less 

 rapidly. This half is invaginated, so that a gastrula arises. The 

 larva which leaves the egg membranes resembles Loven's worm 

 larva in the possession of two eye-spots and a ciliated ring, and 

 develops without a larval shell. 



Fam. Chitonidae. In place of the shell, eight calcareous pieces are present, 

 which are so arranged that the hinder edge of one shell piece overlaps tkft 

 anterior edge of the next following piece. 



Chiton squamosus L., Mediterranean. Cryptocliiton Stelleri, Midd. 



Sub-order 2. Cyclobranchiata. Prosobranchiata with flat plate- 

 shaped shell and foliaceous gills, which are arranged in a closed circle 

 under the edge of the mantle round the broad root of the foot. The 

 buccal lobes are little developed. The foot is powerful, and usually 

 flat and broad. The lingual armature, like that of the Placophora, 

 is formed of toothed horny plates, hence the name Docoglossa 

 of Troschel. A cervical gill placed on the right side of the neck 

 is sometimes present (Lottia). Two kidneys are present. External 

 copulatory organs absent. They feed on plants. 



Fam. Patellidae, (Limpets). The shell is bowl-shaped, and consists of a single 

 piece, to which the animal is attached by a horse-shoe-shaped muscle. Head 

 with two tentacles, at the swollen base of which are placed the eyes. Tongue 

 extraordinarily long and spirally coiled. The radula is without the median 

 plates, while the intermediate and marginal plates are raised to hooks, and 

 smaller lateral plates appear. 



Patella L. The apex of the shell is slightly eccentric, and hardly inclined 

 to the front. P. ccentlea L., P. tarentina Lam., P. scutellaris Lam., Adriatic 

 and Mediterranean. Nacella Schum. Circle of gills broken on the head ; the 

 apex of the pellucid shell, shining internally like mother-of-pearl, bent forwards. 

 N. pelhtcida L. 



Sub-order 3. Zeugobranchiata. Gills bipennate, paired and sym- 



