SEA-MA T MAKERS. 



293 



to be furnished at its origin with long ciliated ten- 

 tacles, and to have a single nervous ganglion situated 

 on one side of the oesophagus (fig. 56). 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. Further, let us imagine the 

 digestive tube, thus constituted, to 

 be suspended in a fluid, contained 

 in a membraneous sac, with two 

 openings, one for the mouth and 

 the other for the vent, the ten- 

 tacles 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, ac- 

 cording to the will of the animal, 

 the retraction being accompanied 

 by an invagination (or folding inwards) 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, occupying some portion of the in- 

 terior of the sac, and the negative character of the 

 absence of all vestige of a heart, we shall have, per- 

 haps, as correct an idea as can be conveyed of the 

 essential structure of a Polyzoon, in its simplest and 

 most generalised condition." 



FIG. 56. 

 TYPICAL POLYZOON. 



