MOLLUSCOIDA : POLYZOA. 281 



soma of a simple member of the class. The term polype is 

 applied to a simple Actinozoon, or to the zooids of a compound 

 actinosoma. Lastly, the t&nn polypide is exclusively employed 

 to designate the zooid of one of the Polyzoa. 



The construction of a typical polypide of a Polyzoon is thus 

 described by Professor Allman (fig. loi, 2) : — 



" Let us imagine an alimentary canal, consisting of oeso- 

 phagus, stomach, and intestine, to be furnished at its origin 

 with long ciliated tentacula, and to have a single nervous gan- 

 glion placed upon one side of the oesophagus. • Let us now 

 suppose this canal to be bent back upon itself towards the 

 side of the ganglion, so as to approximate the termination to 

 the origin. Let us further imagine the digestive tube thus 

 constituted to be suspended in a fluid contained in a mem- 

 branous sac with two openings, one for the mouth and the 

 other for the vent, the tentacula alone being external to the 

 sac. Let us still further suppose the alimentary tube, by means 

 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 entiirely to include the orall 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, ganghon, 

 muscles, generative organs, circumambient fluid, and investing 

 sacs, repeating itself by gemmation, and thus prpducing 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. 



