200 SYNOPTIC COLLECTION. 



called body. 1 It has tubes or siphons which are separate 

 throughout their whole extent, and a foot with a cleft. 



A peculiar bivalve with a body larger than its shell is 

 illustrated by Cyrtodaria siliqua Daudin (No. 373). One 

 preparation shows the remarkably large muscular siphon 

 extended at the posterior end and the comparatively small 

 foot at the anterior end. The other preparation is the 

 fleshy animal taken from its shell. The plump, rounded 

 body contains most of the internal organs. There are 

 two gills, one on either side of the body, and each gill 

 consists of two leaves. 



Mya arenaria Linn. (Pis. 374, 376 ; No. 375), is one 

 of our commonest shells. Ryder 2 has shown that the 

 young clam is attached by a mass of threads called a 

 byssus, but probably only for a short time. The two 

 valves in youth and maturity are equal, therefore the 

 shell is equivalved (PI. 374; No. 375), but the anterior 

 end is broader than the posterior (PL 374) . The light 

 brown external horny layer is thin and usually worn off, 

 showing the lines of growth which are the edges of the 

 layers that make up the shell. The left valve (PI. 374, 

 valve on the left) is provided with one tooth (instead of 

 three as is usually the case), and the right valve (PI. 374, 

 valve on the right) with a cavity which contains the inter- 

 nal ligament. The impressions of the two muscles, the 

 mantle, and the siphon, are more distinctly seen in shells 

 of Mactra (see Nos. 411-413). 



The clam is a differentiated member of its order. Since 

 the internal organs are also better seen in the larger genus, 

 Mactra, we will speak briefly of them here. The mantle 

 has become a sac-like organ with three openings, two at 

 the end of the siphon and one at the anterior end through 

 which the foot passes. The mantle is thickened on the 

 edge and supplied with pigment cells which produce many 

 of the colors of the shell. 



1 Bull. Mus. Comp. Zool., XII, 1886, p. 306. 



2 Amer. Nat., XXIII, 1889, p. 65. 



