506 



GENERAL STRUCTURE OF ALCYONIAN POLYPES. 



sider, the chief subdivisions of this Order, which includes many 

 very interesting and well-known forms. These subdivisions are 

 principally founded upon the nature of the polypidom ; and it is 

 desirable, before proceeding to describe them, that the relation 

 of the parts of which the animals are composed should be clearly 

 understood. Delicate as is the membrane of the Polypes them- 

 selves, it may be distinguished, like that of the Sertularians 

 ( 1177), into two layers. Of these, the outer one is continuous 

 with the general envelope of the whole mass ; whilst the inner 

 one lines the canals which ramify through it. But instead of 

 these two membranes being in contact, as are the horny sheath 

 and the lining of the tubes in the Sertularians, a thick mass 

 of flesh is interposed between them ; and it is usually in this 

 flesh, and not in either of the membranes, that the hard deposit 

 takes place, which gives firmness and support to the general 

 structure. 



1153. The family we shall first consider, includes those 

 species which, have a spongy polypidom, of which the Alcyo- 

 nium is a characteristic example. Here the general form and 



aspect closely re- 

 semble those of the 

 Sponges ; but, in the 

 living state, it will 

 be evident that the 

 projecting orifices are 

 tenanted by Polypes ; 

 and, even in the dry 

 skeleton, it may ge- 

 nerally be observed 

 that the openings of 

 the large canals are 

 not simple, like those of 

 the vents of Sponges, 

 but present some in- 

 dications of the radiating partitions already mentioned. Between 

 the large canals, into which the Polype-stomachs open, there is 

 a net-work of minute tubular ramifications, which connect them 



PIG. "16. ALCYONIUM ; A, portion enlarged, 

 showing the Polypes. 



