236 CLASSIFICATION OF ANIMALS 



surface a thin fold of the ectoderm projects inward toward the 

 principal axis, forming a circular shelf. This is the velum, 

 which serves to distinguish the medusae of this group from the 

 Scyphomedusae. 



526. From the gastric cavity four radial canals run out to 

 the margin of the bell and there join a canal which runs cir- 

 cularly along its edge. In some cases there are six, eight or 

 more radial canals. The gonads are developed from the ecto- 

 derm somewhere along the course of the radial canals or on the 

 manubrium. Special sense organs, eye spots or statocysts, may 

 occur along the edge of the bell and there is also a ring of nerve 

 fibres. The animal has a feeble power of locomotion, effected 

 by a rhythmical contraction of the edge of the bell. The hydro- 

 medusae vary in size from a small fraction of an inch to two 

 inches. In one group they are somewhat larger. Although so 

 different in appearance the medusae are in reality of essentially 

 the same structure as the polyps. They are a little more highly 

 developed in accordance with the free life habit. 



527. When the eggs are fertilized a free-swimming ciliated 

 larva is produced. This becomes attached by the aboral pole, 

 develops tentacles and thus forms a polyp and by budding of 

 the polyp a colony is developed. There is frequently more 

 than one kind of polyp found in a colony. In this case there 

 is a division of function so that there may be feeding polyps 

 which are of the typical form; protective polyps, without ten- 

 tacles and mouth but well supplied with nettling cells; repro- 

 ductive polyps which produce the medusa buds for the colony. 

 Sometimes the medusae remain attached to the parent colony 

 and there mature the reproductive cells. In this case the me- 

 dusa is more or less rudimentary. Such medusae are then 

 another type of polyp and the alternation of generations 

 resolves itself into a special case of polymorphism. 



Order i. The Hydroidea comprise solitary polyp forms which have no 

 medusa stage, like Hydra; the Trachymedusae which have no polyp stage; 



