14 Invertebrate Zoology. 



The Digestive System. Nutritive material entering 

 the mouth is reduced to a semi-fluid condition, and, by 

 the action of the endodermal cilia lining the body-cavity 

 of the hydranth and hydrocaulus, is carried to the various 

 portions of the colony, where it is appropriated and digested 

 by the individual endodermal cells. 



The Circulatory System is not specially differentiated 

 from the digestive system. 



The Muscular System. At certain points in the ten- 

 tacles of favorable specimens, elongated " muscle fibres '' 

 have been described. They lie on the outer surface of the 

 supporting layer. 



The Nervous System. In the present stage of the 

 Hydroid a nervous system has not been found. 



The Reproductive System. The fertilized ovum of a 

 Hydrozoan ordinarily develops into a colony of zooids. 

 Such a colony we have just been considering. Through a 

 process of branching, new colonies are asexually produced, 

 and from the colonies medusa-buds are also produced. 

 A brief search will probably reveal the presence of several 

 gonangia or specialized hydranths. Each gonangium, some- 

 what larger than a hydranth, contains a fleshy central axis or 

 dlastostyle, which is morphologically equivalent to the body 

 of a hydranth. The blastostyle is produced distally into 

 a manubrium. Is there a mouth? Are there tentacles? 

 Do you find the same layers that were present in the 

 hydranth ? Is there a central body-cavity ? Along the side 

 of the blastostyle are medusa-buds, in varying stages of de- 

 velopment. Which are larger, the terminal or those nearer 

 the base ? How many are there ? 



Draw a gonangium with its contents. 



