Pelecypoda 29 



but a number of species have been found to exhibit alternately 

 male and female phases of reproductivity. 



Bivalve mollusks generally bury themselves in sand or mud. 

 Members of some genera bore into wood, concrete, shells, or coral 

 — into almost any nonmetallic substance — and remain imprisoned 

 in these burrows for the duration of their lives {Teredo, Martesia). 

 A few genera, as Modiolaria, attach themselves to the tissues of 

 other animals, though never as parasites, while others are truly para- 

 sitic. Ano7nia, Chama, Ostrea, and Spondylus are attached by one 

 valve. Pecten and Lima, by a rapid flapping of the valves of their 

 shells, expel a jet of water with sufficient propulsive force to drive 

 themselves through the water for considerable distances. 



The Pelecypoda have shells of two joined valves which enclose 

 and protect their soft bodies. In some cases the relative size of the 

 mollusk to its shell is disproportionately large — as in Cyrtopleura — 

 and in other instances the valves are rudimentary, and calcareous 

 tubes are secreted about the elongated body, as in Teredo. The two 

 valves of the pelecypod shell are joined together by a hinge and main- 

 tained in apposition by strong muscles attached to the inner surface 

 of each valve at opposite points. Like the gasteropod shell, the shape 

 of the pelecypod shell is that of a modified cone, flattened from side 

 to side and variously distorted in other dimensions. The apex of 

 each valve is the beak or umbo; the umbos generally point forward 

 with the tips close together and are usually anterior to the hinge 

 ligament. In a few genera, they are directed backward, notably Donax 

 and Nucida. 



An equilateral shell has the umbos at or near the center of the 

 upper margin of the shell (Spisula). An inequilateral shell has the 

 umbos much nearer to one end of the shell than to the other (Area). 

 The terms equivalve and inequivalve refer to the relation of the 

 valves to each other in regard to size. The dorsal margin is that 

 which bears the hinge and umbos. The ventral margin is directly op- 

 posite the umbos, while the anterior and posterior margins are, res- 

 pectively, the front and hinder ends of the shell. 



Most authorities measure the length of a bivalve shell by a 

 line from the extreme anterior margin to the extreme posterior 



