BRACHIOPODA 



171 



whole animal is about 2 "5 mm. long, and about the same 

 in breadth. 



The shells are secreted by the body-wall or by the 

 mantle. Siace the body Ues chiefly in the dorsal shell, the 

 larger part of the latter is secreted by the body-wall, and the 

 dorsal mantle is of small extent; on the other hand, the 

 greater part of the ventral shell is lined by a fold of integu- 

 ment, the ventral mantle. 



The substance of the shell is composed of minute calcare- 

 ous spicules kept together by a network of organic fibrils 

 (Kg. 107). The shell is pierced by numerous canals, whose 

 outer ends are somewhat enlarged and covered with a cuticle. 



Since the mantle is formed of a duplicature of the body- 

 wall, it is necessarily double, and the body-cavity extends into 

 it, in some places this space lodges the reproductive organs. 

 The mantle sends a prolongation into each of the canals in the 

 shell, which is continuous with some of its blood-vessels. These 

 prolongations contain blood- 

 corpuscles, and doubtless 

 serve to nourish the organic 

 fibrils which keep together 

 the calcareous spicules of 

 the shell. The lophophore 

 occupies a considerable part 

 of the dorsal shell, and forms 

 a large part of the body- 

 wall. Its shape is oval, its 

 border running parallel to 

 the edge of the shell, except 

 at the anterior median line, 

 where a narrow deep in- 

 dentation almost divides it 

 into two, and thus gives 

 it a somewhat horse-shoe 

 shape. The indentation is 

 occupied by a median ridge 

 in the dorsal shell (Fig. 107). The lophophore carries 

 round its edge, on the dorsal side of the mouth, from 

 70 to 100 tentacles; at the base of the tentacles is a ciliated 



PiQ. 108. 



Wdtdheimia Jlavescens. Interior 

 of dorsal valve, to show the position of the 

 lophophore. A portion of the fringe of 

 cirrhi has been removed to show the 

 brachial membrane, and a portion of the 

 spiral extremities of the arms. 



A. Position of mouth. 



