MOLLUSCOIDA : POLYZOA. 



26l 



The two fundamental structures of the " coenoecium " of a 

 Pulyzoon viz., the immediately investing sac, and its secon- 

 dary investment, are sometimes termed the " endoderm " and 



J 



g 



Fig- 94- Morphology of Polyzoa. i. Portion of the coenoecium of Flustra truncata, 

 magnified. 2. Diagram of a Polyzoon (after Allman): a Region of the mouth 

 surrounded by tentacles; b Alimentary canal; c Anus ; d Nervous ganglion; e 

 Investing sac (ectocyst) ; /Testis ;/' Ovary ; g Retractor muscle. 3. Bird's-head 

 process, or " avicularium," of a Polyzoon. 



" ectoderm ;" but as these terms are employed in describing the 

 Hydrozoa, it is better to make use of the terms " endocyst " 

 and "ectocyst," proposed by Dr Allman. 



The " ectocyst," or external investment of the ccencecium, 

 is usually a brown, pergamentaceous, probably chitinous, but 

 often highly calcareous, membrane ; and it is by the ectocyst 

 that the "cells" are formed. In Cristatella, alone of the 

 Polyzoa, there is no ectocyst, and in Lophopus (fig. 95, 3) the 

 ectocyst is gelatinous in its consistence. In many cases the 

 ectocyst is provided with singular appendages, supposed to be 

 weapons of offence and defence, termed " avicularia " (fig. 94, 

 3) and "vibracula." The avicularia, or "bird's-head processes," 

 differ a good deal in shape, but consist essentially of "a 

 movable mandible and a cup furnished with a horny beak, 

 with which the point of the mandible is capable of being 

 brought into apposition." (Busk.) In shape the avicularia 

 often closely resemble the head of a bird, and they are in many 

 respects comparable with the " pedicellarise " of the Echinoder- 

 mata* In the " vibracula," the place of the mandible of the 



* There is great reason, however, as shown by Huxley, to regard the 



