124 



COLLEGE ZOOLOGY 



individual producing either eggs or spermatozoa. These repro- 

 ductive cells break out directly into the 

 water, where fertilization takes place. 

 A ciliated planula develops from the 

 egg as in Obelia (Fig. 73, C). This 

 soon becomes fixed to some object, and 

 a mouth appears at the unattached 

 end. Then four tentacles grow out 

 around the mouth and the Hydra-like 

 larva is able to feed (Fig. 75). Other 

 similar Hvdra-likc larvae bud from its 



V 



walls. How the medusae arise from 

 these larvae is not known, but it seems 

 probable that a direct change from 

 the hydroid form to the medusa 

 occurs. 



/' 



FIG. 75. Hydralike stage 

 in the development of Gonio- 

 nemus. One of the tentacles 

 is carrying a worm (\v) to 

 the mouth. Tentacles in 

 contracted state. (From the 

 Cambridge Natural History, 

 after Perkins.) 



d. Hydroid and Medusa Compared 



Although the medusae upon superficial examination appear to 

 be very different from the polyps or hydroids, they are con- 



fmf 



rod 



cn:c 



FIG. 76. Diagrams showing the similarities of a polyp (A) and a medusa 

 (B). circ, circular canal; eel, ectoderm; end, entoderm; ent. cav, gastrovascu- 

 lar cavity; hyp, hypostome; mnb, manubrium; msgl. mesoglea; mth, mouth; 

 nv, nerve rings; rad, radial canal; v, velum. (From Parker and Haswell.) 



