SOS' 



Mollusca. 



is, however, somewhat thioker, and provided with transverse muscle 

 fibres. The upper ends of these fibres are firmly attached to the 

 so-called pallial line {s^e below) on the inner side of the shell, and 

 by their contraction withdraw the margin of the mantle within its 

 edge : along the free edge occur numerous tactile tentacles. The 

 edges of the two mantle lobes are perfectly free from each other; 

 in a few forms only (e.g., the Oysters), as a rule, they are partially 

 concrescent, so that the large opening between them is divided 

 into two or several divisions. In the simplest cases {e.g., in the 

 Mussels), only the most posterior portion is separated from the rest, 

 the edges of the mantle being connected for a short distance; in 



such there is a small pos- 

 terior opening, the cloacal 

 aperture, through which 

 water and excreta escape 

 from the mantle-cavity, 

 and a very large open- 

 ing through which water 

 streams in, and the foot 

 can be protruded. In 

 others (Pig. 264) the large 

 opening just mentioned is 

 divided into an anterior 

 large aperture for the foot, 

 and a posterior smaller pore 

 for the inhalent current of 

 water, the mantle-lobes having concresced for a longer or shorter 

 distance ;* thus the originally single slit is divided into three : 



posteriorly the cloa- 

 "^^^"'''Ssv cal aperture, below 



this the inhalent 

 aperture, through 

 which the water 

 streams in, and an- 

 terior to, and below, 

 this, the pedal ori- 

 fice, through which 

 the foot may be pro- 

 truded, the last being 

 much larger than the 

 other two. The cloacal 

 and inhalent apertures 

 are often drawn out 

 into two tubes, the 

 cloacal and r e s - 



Fig. 254. ALamellibrancli removed from the 

 shell, seen from the left side, a and ft points of 

 concrescence along the edge of the mantle, r left, 

 r' right edge of mantle, / foot, m and m^ ends of the 

 adductors. — Orig. 



Pig. 255. A LameUibranoh (Tapes decussatus) with 

 partially separated syphons. The arrows indicate the 

 direction of the streams of water, n foot. 



* The edges of the mantle may lie closely 

 lUidergoing ooBQrescenoe. 



apposed is tbege regions without 



