HYDROZOA. 25 



minute cells or sacs, " probably composed of some 

 tenacious mucus with which to glue the egg to 

 any substance on which it may happen to settle ; " 

 an inference supported by the fact, that, soon after 

 the attachment of the egg, they disappear. Each 

 spermatozoon consists of an oval body, furnished 

 with a very delicate tail. The act of fecundation 

 most probably takes place after expulsion of the 

 ovum. 



2. General morphology. — Simple in struc- 

 ture as is the Hydra, it must, nevertheless, be 

 viewed as the representative of an extensive assem- 

 blage of animal forms, whose outward aspect is 

 singularly diversified, whilst, at the same time, 

 the utmost uniformity prevails throughout all the 

 more essential features of their organisation. In 

 every Hydrozoon, the wall of the digestive appa- 

 ratus coincides, or is continuous, with that of the 

 somatic cavity, and the entire body, or ' hydro- 

 soma,' howsoever modified, is resolvable into ecto- 

 dermal and endodermal layers, processes of one 

 or both of which constitute, in like manner, the 

 manifold and complicated appendages, frequently 

 superadded (Jig. 5, b). 



One end of the hydrosoma is always found to 

 increase more quickly than the other, which in 

 some cases, though not in all, is absolutely fixed. 

 The term ' distal ' is .employed to distinguish this 

 rapidly growing end from the opposite, or ' proxi- 

 mal,' extremity. 



In Hydra, and a few of its more immediate 

 allies, the hydrosoma consists, as has been shown, 

 of an alimentary region, or ' polypite,' together 



