10 



THE CRAG POLYZOA. 



Fig. 2. 

 a 



OF THE CHEILOSTOMATA. 



The members of this sub-order may perhaps be regarded as the most highly organized 

 of the Polyzoa, inasmuch as in addition to the common attributes of the class, 

 they are provided with certain organs not found in their allies. The points in 

 which they differ from the Cyclostomata are I, in the form of the cell, which is never a 

 simple, open, cylindrical tube of pretty nearly uniform diameter throughout, but is either 

 urceolate, turbinate, or of an elliptical or oblong figure, or, as in ^Etca, tubular, expanded, 

 and closed at the extremity ; 2, in the position of the orifice, which is never strictly 

 terminal, but always either parallel or oblique to the axis of the cell, and sometimes placed 

 near the centre of the front ; 3, by the circumstance that the orifice is fitted with a 

 special apparatus for its closure when the animal is retracted. In the greater number of 

 cases this purpose is effected by means of a membranous or horny, 

 semicircular lip or shutter, the two angles of which are articulated 

 to the sides of the orifice, and which is moved by special muscles (fig. 2). 

 Although from its extensive prevalence the sub-order lias derived its 

 name from the presence of this organ, it does not exist universally, 

 the closure of the orifice in some cases, as in Bugula, being effected, 

 as it would seem, by a sort of muscular sphincter instead. 



The consistence of the cell-walls in the Cheilostomata varies 

 from a soft fleshy, or a horny texture, to one either wholly or partially 

 calcareous. But in many of the instances in which it is usually 

 regarded as altogether corneous, or in which, at any rate, a conside- 

 rable degree of flexibility is retained, incineration will show that a 

 sufficient amount of calcareous matter is present to maintain the form of the cell, though 

 of course in a most fragile condition, as is the case, for instance, in many Flustree, &c. 



In a great many, moreover, of the more highly calcified Cheilostomata, the walls of 

 the cell remain to some extent membranous. In these cases, when the animal matter is 



removed, a greater or less space in or the whole of the anterior 

 surface will appear open. This space (fig. 3 b) in the recent condition 

 is filled up by a thin membrane, in which, in some instances, the 

 ,, orifice with its moveable lid, or surrounded with its sphincter, is 

 placed. In many species also, belonging to the encrusting forms 

 the posterior wall of the cell is apparently altogether deficient, 

 the cavity being bounded behind by the surface of the shell, 

 &c., upon which the cell is affixed ; although it cannot be doubted 

 that a membranous pellicle must intervene between this surface 

 and the perivisceral space of the Polyzoon. 



o. Orifice. 



i.. 



