204 STRUCTURE OF THE MOLLUSCA. 



The LingulcE have a long fleshy stalk, of which one end 

 is generally tixed to a rock, and the other bears two oblong, 

 flattened valves. Their arras, which are attached on the 

 sides of the mouth, are very long; and the branchial vessels 

 are dispersed over the inner surface of the mantle, forming 

 on each side a series of small parallel folds. 



The TerehratuJce have two unequal valves joined by a 

 hinge; and it is through a hole at the summit of one of 

 them that the fleshy peduncle passes by means of which the 

 animal is fixed. Their branchiae are less distinct than those 

 of the Lingulse, and consist of a network of vessels on the 

 inner surface of the mantle ; but their muscular system is 

 more developed, and there is in the interior of the shell a 

 small solid apparatus, of which the structure is sometimes 

 very complex, and of which the principal use is to furnish 

 points of attachment to the muscles, and to assist in sepa- 

 rating the valves. 



The OrbiculincE have a round and conical valve, like that 

 of the Patellae, while the other is flat, and perforated for the 

 passage of a very small pedicle. Their arms and branchial 

 vessels resemble those of the Terebratulae. 



CLASS YI -TUNTCATA. 



The Tunicated or acephalous shell-less Mollusca, fig. 7, 

 differ much from all the rest in their general form, as well 

 as in several important peculiarities of their structure. Their 

 mantle forms a bag, fig. 7, a, or a tube open at both ends. 

 Their branchiae are always small, and never divided into 

 four laminae. They have neither foot nor arms, and evi- 

 dently form a passage to certain lower animals which are 

 placed among the polypi in the division of the Zoophytes. 

 Their structure is more simple than that of any other Mol- 

 lusca, and they live for the most part affixed to submarine 

 bodies. 



