406 POLYZOA. 



ing mediuQi, or coenosarc, from which they take their origin." 

 The accompanying sketch of Flustra foliacea (one of the Sea 

 Mats, Plate XVIII., Fig. i) will illustrate the foliaceous polyzoa, a^ 

 representing a portion of the colony, natural size, and b, a fragment 

 magnified to show the cells in which the separate polypides are 

 contained. Fig. 2 is the diagram of a Polyzoon (after Allman) : 

 «, Region of the Mouth surrounded by tentacles ; b, Alimentary 

 canal ; c^ Anus ; d^ Nervous ganglion ; f, Investing sac (ectocyst) ; 

 f^ Testis ; f\ Ovary ; g^ Retractor muscle. 



I cannot describe its construction better than by giving the 

 exact words of Professor Allman. " Let us imagine," writes this 

 able naturalist, " an alimentary canal, consisting of oesophagus, 

 stomach, and intestine, to be furnished at its origin with a long, 

 ciliated tentacula, and to have a single nervous ganglion 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 membranous 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 protruded 

 according to the will of the animal, the retractation being accom- 

 panied by an invagination of the sac, so as partially 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 considera- 

 tions of homology or derivation from an archetype — as can be 

 conveyed of the essential construction of a polyzo5n 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 differ- 

 ent groups ; and we may, moreover, conceive of the entire animal 



