ANATOMY AND PHYSIOLOGY. 



1. 



Preliminary Matter. 



General conception of a Poli/zoon. 



Fig.l. 



Fig. 2. 



Plan of a Polyzoon. 

 Fig. 1. Exserted. Fig. 2. Retracted. 



o. Region of mouth, surrounded by tentacles, b. Alimentary canal, c. Anna. d. Nervous ganglion. 

 e. Membranous investing sac. /. Testis. /'. Ovary, g. Retractor muscle. 



Let us imagine an alimentary canal, consisting of oesophagus, stomach, and intestme, to be 

 furnished at its origin with long ciliated tentacula, and to have a single nervous ganglion 

 situated on 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 appro.ximate the termination to the origin. 

 Further, let us imagine the digestive tube thus constituted to be suspended in a fluid con- 

 tained 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 retraction being accompanied by an invagination of the sac so as 

 partially or entirely to include the oral tentacula 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 theoretical considerations of 

 homology or derivation 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 



