STRUCTURE OF POLYZOA. 



577 



plane surfaces, as is the case with Flustrce, Lepralice, he. (Fig. 

 30U) ; whilst not (infrequently the Polyzoary developes itself into 



Fig. 300. 



Cells of Lepralice :— a, L. Hymhnanni ; b, L.figularis; c,L. verrucosa. 



an arborescent structure (Fig. 301), which may even present 

 somewhat of the density and massiveness of the Stony Corals. 

 Each individual, designated as a Polypide or polype-like animal, 

 is composed externally of a sort of sac, of which the outer or 

 tegumentary layer is either simply membranous, or is horny, or in 

 some instances calcified, so as to form the Cell ; this investing sac 

 is lined by a more delicate membrane, which closes its orifice, and 

 which then becomes continuous with the wall of the alimentary 

 canal ; this lies freely in the visceral sac, floating (as it were) in 

 the liquid which it contains. 



446. The principal features in the structure of this group will 

 be best understood from the examination of a characteristic ex- 

 ample, such as the LaguncuJa repens : which is shown in the state 

 of expansion at A, Plate xxn., and in the state of contraction 

 at B and c. The Mouth is surrounded by a circle of tubular Ten- 

 tacula, which are clothed with vibratile cilia ; these tentacula, in 

 the species we are considering, vary from ten to twelve in number, 

 but in some other instances they are more numerous. By the 

 ciliary investment of their tentacula, the Polyzoa are at once dis- 



p P 



