VI 



INTRODUCTION. 



(Woodcut, fig. ii, sh). All but universally* the alimen- 

 tary canal is furnished with a system of muscles, by 

 means of which it can be partially exserted and re- 

 tracted. 



Fig. ii. 



t 



■int- 



sk. Tentacular sheath (invaginated). t. Tentacles, gr. Grreat retractor 

 muscles, st. Stomach, oes. CEsophagus, doubled up alongside the stomach. 



Within the cavity of the cell reproductive organs are 

 developed, in various positions. Heart and vascular sys- 

 tem are wanting. 



These are the general features of the polyzoan type of 

 structure. 



The ZofficiUMf. 

 The zooecium or cell, the home of the polypide, and one 

 of the elements of which the coenoecium is composed, is 

 the primary zooid of the colony, and as such is entitled to 

 our first consideration. In its normal condition it is a 

 sac, of variable size and shape, which incloses and protects 



* The exceptions here also are the Entoprocta, in which the polypide is 

 not retractile and the tentacles are merely rolled up, and Rhahdopleura, in 

 which the muscular system is suppressed and the polypide crawls out of its 

 cell by means of a large foot-like organ. 



t 7mov, animal, and oIkIov, house. 



