282 



MANUAL OF ZOOLOGY. 



The two fundamental structures of the " ccenoecium " of a 

 Polyzoon — y\z., the immediately investing sac, and its secon- 

 dary investment, are sometimes termed the " endoderm " and 



Fig. loi. — Morphology of Polyzoa. i. Portion of the coenoecium oi Flitstra intncataf 

 magnified. 2. Diagram of a PolyzoOn (after AUman): ci Region of the mouth 

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

 Investing sac (ectocyst) ; /Testis ;y' Ovary J ^Retractor muscle. 3. Eird's-head 

 process, or " avicularium, of a PolyzoSn. 



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

 Hydrozoa, it is better to make us& 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. 102, 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. loi, 

 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 " pedicellariae " 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 



