MOLLUSCOIDA I POLYZOA. 



235 



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 derivation 

 from an archetype as can be conveyed of the essential struc- 

 ture 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.' 



Fig. 72. Morphology of Polyzoa. 1. Portion of the coenoecium of Flustra 

 truncata, magnified. 2. Diagram of a Polyzoon (after Allman) ; a. 

 Region of the mouth surrounded by tentacles ; 6. 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. 



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

 universally the case. Thus the singular fresh- water Gristatella 

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

 discoid base, not unlike the foot of the Gasteropoda. 



The two fundamental structures of the ' ccencecium ' of a 



