308 COMPARATIVE ANATOMY. 



A small number of ciliated tentacular processes are developed in 

 the larvae at the sides of the mouth. In the mature stage of the 

 animal they are generally arranged on a stalk, which can be rolled 

 up in a spiral fashion ; there is a large number of them, and they are 

 placed on either side of the mouth. When these arms are rolled 

 up they are placed in the anterior portion of the mantle-cavity 

 (Fig. 166, /) ; they appear to be erected by injection of blood. 

 Owing to the great development of these arms, and of the mantle- 

 folds, the rest of the body is reduced to a small size ; and even the 

 organs which, in other cases, lie in the ccolom, may be embedded in 

 the folds of the mantle (pallial cavity). The mantle acquues a 

 respiratory significance when the inner lamellae of its folds are 

 increased in surface by the development of ridges ; it then functions 

 as a gill (Lingula). 



The tentacular processes around the mouth call to mind the 

 tentacles of the Bryozoa, which may also be arranged on arm- 

 like structures (lophophore) ; but they can no more be com- 

 pared with these structures than with the branchial tufts of the 

 Tubicolas. 



Finally, the stalk is, in the older forms (Lingula), a long portion 

 of the body, which passes out between the two shells, and is 

 movable, while in the Testicardines it is short and largely chitinrsed. 



Integument, Shell, and Arms. 

 § 240. 



As the two shells cover all the body except the stalk, the parts 

 of the surface of the body are only free within the pallial cavity, and 

 are only exposed when the shell is open. From the fact that muscles 

 are connected with the integument, we may suppose that there is a 

 dermo-muscular tube. Calcareous spicules, which distinguish the 

 integument, are found in the mantle, as well as in the arms. They 

 are sometimes branched, but may be stellate or form a network. 

 The setas, which beset the edge of the mantle in various ways 

 in various genera, are of more significance. Like the setaa of 

 the Chajtopoda they are formed in glandular depressions of the 

 integument, and, like them, are cuticular formations. They are 

 generally simple structures, which terminate by a fine point, and 

 indicate by their transverse striation that they have been gradually 

 secreted. 



In the Ecardines the two valves of the shell have pretty much 

 the same form. But in the Testicardines the dorsal and ventral 

 valves are distinctly differentiated. Towards the stalk they are 

 united by a kind of clasp. The ventral valve also is drawn out into 

 a beak-like process, the punctured end of which serves as a passage 

 for the stalk. A skeleton, which projects inwards, is developed 



