374 MANUAL OF ZOOLOGY. 





truded according to the will of the animal ; the retraction 

 being accompanied by an invagination of the sac, so as par- 

 tially or entirely to include the oral tentacles within it ; and if 

 to these characters we add the presence of true sexual organs 

 in the form of ovary and testis, occupying some portion of the 

 interior of the sac, and the negative character of the absence 

 of all vestige of a heart, we shall have, perhaps, as correct 

 an idea apart from all considerations of homology or deriva- 

 tion from an archetype as can be conveyed of the essential 

 structure of a Polyzoon in its simplest and most generalised 

 condition. 



" To give, however, more actuality to our ideal Polyzoon, 

 we may bear in mind that the immediately investing sac has 

 the power, in almost every case, of secreting from its external 

 surface a secondary investment, of very various constitution 

 in the different groups; and we may, moreover, conceive of 

 the entire animal with its digestive tube, tentacula, ganglion, 

 muscles, generative organs, circumambient fluid, and investing 

 sacs, repeating itself by gemmation, and thus producing one 

 or more precisely similar systems holding a definite position 

 relatively to one another, while all continue organically united, 

 and we shall then have the actual condition presented by the 

 Polyzoa in their fully-developed state." 



The vast majority of the Polyzoa are fixed, but this is not 

 universally the case. Thus the singular fresh-water Cristatella 

 is free and locomotive, creeping about by means of a flattened 

 discoid base, not unlike the foot of the Gasteropoda ; and the 

 polyzoary seems to have been unattached in a few other forms 

 (Selenaria, Cupularia, &c.) 



The two fundamental structures of the " coencecium " of a 

 Polyzoon viz., the immediately investing sac, and its second- 

 ary investment are sometimes termed the "endoderm" and 

 " 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 coenoecium, 

 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. 199, 3) 

 and in the curious Pectinatella 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, or organs of prehension, termed " avicularia " (fig. 

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



