THE PHYLA COELENTERATA AND CTENOPHORA 



189 



B. Polyp 



A. Planula. 

 Figure 10.7. Reproduction in Gommiemiis. A, Planula larva that develops from 

 the egg. B, 1 he polyp, showing mouth and four tentacles. A frustule is forming on the 

 side and is also shown in successive stages as it later creeps away. {A after Hyman; 

 B modihed from Hyman after Joseph.) 



One side ot the bod) thickens, becomes constricted as a separate tube, 

 and very slowly creeps away. This bud or frustule has no mouth, 

 tentacles or stomach cavity. Over the span of several days the frustule 

 may move several inches, after which it settles do^\•n with one end at- 

 tached and develops into a typical polyp. 



Asexual reproduction by budding is connnon among the coelen- 

 terates. Most j^olyp stages are able to reproduce this way but only a few 

 kinds of medusae show the phenomenon. We have observed that sponges 

 reproduce by asexual buds, and as we shall see later many other animal 

 groups do also. Coelenterates may pass through many generations of 

 asexual budding before developing sexually mature individuals. 



In the summer Gouiotieiniis polyps produce spherical buds that 

 develop into medusae. A well fed polyp may produce several such buds 

 but a small or starved individual may produce only one. In the latter 

 case the entire polyp may transform into a medusa. 



While still attached, the medusa bud develops a velum, manubrium 

 with mouth, and eight tentacles. It begins to pulsate and eventually 

 breaks free by its own ac tivity. As it grows, increasing its diameter from 

 1 mm. to 2 cm., new tentacles grow out between those already present 

 on the umbrellar margin. 



65. Classes of the Phylum Coelenterata 



Differences in structure and life history are the criteria for grouping 

 coelenterates in three classes. Gonionemiis belongs to the class Hydrozoa, 

 in which the medusa has a velum and the polyp has a simple, unpar- 



