94 INVERTEBRATE ZOOLOGY 



likened unto a single individual with organ systems resembling 

 those found in higher Metazoa. 



Dimorphism. — A condition of two unlike forms in the same 

 generation is not unusual in the Hydrozoa. This condition is 

 most commonly encountered as the specialization of vegetative 

 and reproductive zooids in the same colony as described for 

 Obelia. Dimorphism of a less common type occurs in the stag- 

 horn coral Millcpora, wherein gasterozooids provided with 

 mouth and tentacles and dactylozooids lacking the mouth are 

 associated in the same colony. Neither of these represents the 

 gonophore generation, for rudimentary jellyfishes of both sexes 

 are produced, usually in chambers within the coral-like skeleton 

 of the colony. 



Histological Differentiation. — As was stated in the introduc- 

 tory discussion of the characters of the phylum, the medusa 

 represents a higher type of differentiation than is found in the 

 hydroid polyp. Scattered nervous elements and sensory cells 

 characteristic of the polyp are replaced by a centralized nerve 

 ring with which statocysts and light-percipient organs are asso- 

 ciated in the hydromedusa except in the Anthomedusae, which 

 are without statocysts. Hertwig has called attention to the 

 fact that no less than eight distinct lines of differentiation occur 

 in the ectoderm cells of the hydromedusa. There are myoblasts, 

 nerve cells, indifferent cells, sensory cells, cnidoblasts, gland 

 cells, pigment cells, and germ cells. Since a number of these 

 are represented by several distinct types of histological differen- 

 tiation, as, for example, the several distinct types of sensory 

 and gland cells, these simple diploblastic animals become more 

 highly organized than is ordinarily understood. 



Body Covering and Skeleton. — An investing membrane 

 surrounds some hydroid forms (Obelia) but is wanting in others 

 (Hydra, Tubularia). This investing membrane may be either a 

 delicate perisarc covering all or only part of the zooid or, in 

 some instances (Millepora) part of a massive calcareous skeleton. 

 The expansion of perisarc around a vegetative zooid is termed a 

 hydrotheca, while that around a reproductive zooid is termed a 

 gonotheca. 



Class Sc3rphozoa 



The Scyphozoa are coelenterates having an alternation of 

 generations of which the medusoid is the more conspicuous. 



