PHYLUM CCELENTERATA 



127 



TABLE VI 



POLYMORPHIC MODIFICATIONS OF THE HYDROIDS OF THE HYDROZOA 



NAME 



STRUCTURE 



FUNCTION 



Gastrozooid 



Dactylozooid 

 Blastostylc 



\Vilh large mouth, nemato- 

 cysls, and tentacle bear- 

 ing nematocysts (Fig. 78, 



e, /) 



Without ' mouth ; with 

 many .nematocysts and 

 tentacle (Fig. 78, g, h) 



Without mouth or tentacles 



Ingestion of food 



Offense and defense 



Produces sexual me- 

 dusoids by budding 



/. Reproduction in the Hydrozoa 



The methods of reproduction differ so widely among the HY- 

 DROZOA that only a brief general account can be given here. 

 Reference should be made to the descriptions for Hydra (p. 115), 

 Obelia (p. 121), and Gonionemus (p. 123). 



Asexual reproduction is characteristic of some HYDROZOA and 

 rare or absent in others. The most common method is by bud- 

 ding (Hydra, p. 115, Fig. 65). The wall of the hydroid sends 

 out a hollow protrusion which may become either a new hydroid 

 or a medusa. Certain medusae also produce medusae by bud- 

 ding. Fission is rare in hydroids (Hydra, p. 116, Fig. 71) and 

 very rare in medusae. 



Sexual Reproduction. - - Both male and female germ-cells are 

 rarely developed by a single individual as in Hydra (Fig. 65). 

 Usually a colony produces either ova or spermatozoa, or these 

 originate in different individuals of a single colony. Sometimes 

 one blastostyle may give rise to both kinds of germ-cells. The 

 development of the fertilized egg has already been described in 

 Hydra (p. 116), Obelia (p. 121), and Gonionemus (p. 124). 



