MOLLUSCOIDA : POLYZOA. 



301 



of a system of muscles, to admit of being retracted or pro- 

 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. 



Fig. 123. Morphology of Polyzoa. i. Portion of the ccencecium of Flustra. truttcata, 

 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) ; f Testis ; _/' Ovary ; g Retractor muscle. 3. Bird's-head 

 process, or "avicularium," of a Polyzoon. 



"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, 



