



Microns 



200 



Figure 84. Cross section of the radial mviscle of an adult C. virginica. The muscle is not accompanied by nerve. Bouin 



3, hematoxylin-eosin. 



and ciliated; those on the outside under tlie valves 

 bear no cilia and are much shorter, in places 

 almost cubical. 



Tlie two sides of the mantle perform differei^t 

 functions. The inner side maintains ciliary cur- 

 rents, whicli in general move from the base of the 

 mantle to its edge and carry mucus and sediments 

 settled from the water; this material is passed to 

 the margin of the shell to be discharged. Tiie 

 epithelium of tlie outer side secretes the inner 

 layer of the shell, the so-called calcito-ostracum. 



Although the ciliated epithelium of the edge of 

 the mantle contains the same kind and proportion 

 of cellular elements found in other parts of tlie 

 organ, the cilia at the b<irder of the mantle are 

 especially powerful. The tentacles themselves 

 consist of a core of connective tissue with associ- 



ated blood vessels, elastic fibrils, and muscle 

 fibers which emerge from branches of the radial 

 muscles. On the outside the tentacles are covered 

 with a single layer of ciliated epithelium to which 

 black or brown pigment imparts a dark color. 

 Special sense organs are absent but the tentacles, 

 especially the long ones, are well supplied with 

 nerves branching out from the nerve which enters 

 tlie base of the tentacle and is itself connected 

 with the nervous system of the mantle (fig. 86). 



The circumpallial nerve provides communica- 

 tion between the tentacles and the radial nerves. 

 The structure of this nerve resembles that of a 

 ganglion: numerous nerve cells of the types found 

 in visceral and other ganglia (see p. 28S) occupy 

 the peripliery of the nerve; its center consists of 

 nerve bundles with occasional small ganglion cells. 



THE MANTLE 



85 



