6o6 



MOLLUSCOIDEA. 



parently always more or less clearly united with one another by an 

 organic connection. In many cases this communication between 

 the different polypides is effected by means of the structure which 

 has been previously spoken of as the " endosarc." In other cases 

 the walls of the cells are pierced by pores or traversed by tubuli, by 

 means of which the perivisceral cavities of adjoining polypides are 

 placed in communication. In other cases, again, there exists a 

 common lamina, the extension of which precedes the production 

 of new cells. Lastly, in the group of Polyzoa known as the 

 Ctenostomata, there exists a common tube with which all the poly- 

 pides of the colony are connected. 



The structures in the Polyzoa with which the palaeontologist is 

 more especially concerned are those developed from the external 



Fig. 451.— A, Fragment of Stomatopora (Alecto) dilataus, a recent Cyclostomatous Polyzoon, 

 enlarged. B, A few cells of Smittia Marionensis, a recent Cheilostomatous Polyzoon, enlarged : 

 ?n, Aperture 'of the cell ; a, Avicularium ; 0, Ovicell. (After Busk.) 



investing layer or " ectocyst," which form the skeleton of the colony, 

 and which alone are capable of being preserved in the fossil condi- 

 tion. In rare cases among the existing Polyzoa the ectocyst may be 

 absent, or may be gelatinous in consistence ; but it is usually either 

 purely chitinous or more or less extensively calcined. The ectocyst 

 forms for each polypide of the colony a more or less complete 

 " cell " ; but the cell is invariably furnished at one point with an 

 " aperture " or " mouth," whence the polypide can protrude its 

 tentaculate head. In various forms, however, the anterior wall of 

 the ectocyst may be imperfectly hardened, or may remain mem- 

 branous. The form of the " cells " or " zocecia " in the Polyzoa 

 varies extremely in different groups, and important distinctions are 



