iv PHYLUM CCELENTERATA 141 



degree of organisation than the polype, having more perfect 

 muscular and nervous systems, distinct sense-organs, and a diges- 

 tive cavity differentiated into central and peripheral portions, the 

 latter taking the form of radial and circular canals. The repro- 

 ductive products are discharged externally, and are very commonly, 

 though not always, of ectodermal origin. 



Many Hydrozoa agree with Obelia in exhibiting alternation of 

 generations, the asexual generation being represented by a fixed, 

 more or less branched hydroid colony, the sexual generation by a 

 free-swimming medusa. In other forms there are no free medusae, 

 but the hydroid colony produces fixed reproductive zooids. In 

 others, again, there is no hydroid stage, the organism existing only 

 in the medusa-form. Then, while in most instances the only 

 skeleton or supporting structure is the horny perisarc, there are 

 some forms in which the ccenosarc secretes a skeleton of calcium 

 carbonate, forming a massive stony structure or coral. Lastly, 

 there are colonial forms which, instead of remaining fixed, swim 

 or float freely on the surface of the ocean, and such pelagic species 

 are always found to exhibit a remarkable degree of polymorphism, 

 the zooids being of very various forms and performing diverse 

 functions. 



Thus we have zoophyte colonies known to produce free medusae, 

 zoophyte colonies known not to produce free medusae, and medusa 

 known to have no zoophyte stage. Moreover, there are many 

 medusas of which the life-history is unknown, so that it is un- 

 certain whether or not a zoophyte stage is present. It is also 

 found that in some cases closely allied zoophytes produce very 

 diverse medusae, while similar medusae, in other cases, may spring 

 from very different zoophytes. For these reasons a sort of double 

 classification of the Hydrozoa has come about, some zoologists 

 approaching the group from the point of view of the zoophyte, 

 others from that of the medusa. On the whole the following 

 scheme seems best adapted for bringing before the beginner the 

 leading modifications of the class. 



ORDER 1. LEPTOLIN.E. 



Hydrozoa in which there is a fixed zoophyte stage, and in which 

 the sense-organs are exclusively ectodermal. 



Sul-Ordcr a. Anthomedit.sce. 



Leptolinse in which the polypes are not protected by hydrothecre or the 

 reproductive zooids by gonotheca? : the medusa? bear the gonads on the manu- 

 brium and have no lithocysts. 



I 



Sub-Order 1). Leptomedusce. 



Leptolinse in which hydro- and gonothecce are present : the medusa? bear the 

 gonacls in connection with the radial canals and usually have lithocysts. 



