486 MOLLUSCA 



part of the coeloin and communicates with the mantle cavity by means of 

 the extensive nephridia (13, 14). The sexes are separate, the gonad com- 

 municating with the mantle cavity by a pair of ducts which join it in 

 front of the nephridial pores. The eggs are sometimes laid in a common 

 jelly which is attached to the rocks, or they may be attached separately 

 or in small groups. Chiton polii retains its eggs in the mantle cavity until 

 they hatch. The young are bom as trochophore larvae.* 



The Polyplacophora are found in all parts of the world, occurring 

 chiefly in shallow water, although a few species live in the deep sea. 

 They attach themselves to rocks, shells, corals, etc., feeding on diatoms 

 and other plants. In the West Indies and other places they are an 

 article of food, the foot being called sea beef. About 600 species are 

 known, grouped in 3 suborders. 



Key to the suborders of Polyplacophora: 



tti Valves without insertion plates 1. EOPLACOPHORA 



fl. 2 All valves with insertion plates. 



&! Margin of insertion plates not pectinate 2. MESOPLACOPHORA 



6, Margin of insertion plates pectinate 3. TELEOPLACOPHORA 



SUBORDER 1. EOPLACOPHORA. 



Valves of large size, the anterior and posterior ones similarly articu- 

 lated; insertion plates absent, except in certain cases on the anterior 

 valve, and unslit: 1 family. 



FAMILY LEPIDOPLEUEIDAE. 



With the characters of the suborder: 4 genera. 

 HANLEYIA Gray. Insertion plates wanting on all valves 

 of the shell but the anterior one, and here they are unslit, 

 but roughened; girdle with spines: 3 species. 



H. mendicaria (Mighels and Adams) (Fig. 747). Shell 

 grayish with a red margin, elliptical ; surface with elevated 

 dots, without concentric striae; gills posterior, about 15 on 

 a side; length 27 mm.; breadth 15 mm.: northern New England, in 

 shallow water. 



SUBORDER 2. MESOPLACOPHORA. 



All the valves with insertion plates, either the first alone or the first 

 7 of which are slit, and with smooth or roughened but not pectinate 

 margins ; eyes absent : 4 families. 



* See "Contributions to the Embryology of Chiton," by M. M. Metcalf, Johns Hop. 

 Univ., Stud, from Biol. Lab., Vol. 5, p. 249, 1893. 



