128 COLLEGE ZOOLOGY 



g. Classification of the Hydrozoa 



The Hydrozoa may be distinguished from the Scyphozoa 

 and Anthozoa by the absence of a stomodaeum and mesen- 

 teries (Fig. 84), and by the fact that their sexual cells are dis- 

 charged directly to the exterior. In classifying the Hydrozoa, 

 both the hydroids and medusae are considered. The arrange- 

 ment adopted in this book is from Fowler in Lankester's Treatise 

 on Zoology. 



Order 1. Anthomedusae. Hydrozoa usually with two 

 forms of individuals, (1) non-sexual fixed hydroids, and (2) fixed 

 or free-swimming sexual medusae. The perisarc (absent in 

 Hydra) does not form hydrothecae around the polyp nor gono- 

 thecae around the reproductive zooids. The reproductive or- 

 gans are in the wall of the manubrium. The hydroids are usually 

 colonial, with solid tentacles in one or more whorls. Examples: 

 Hydra, Hydradinia, Endendrium, Tubularia. 



Order 2. Leptomedusae. — Hydrozoa with an alternation 

 of non-sexual fixed hydroids and free or fixed sexual medusae. 

 The hydrothecae and gonothecae are specialized portions of 

 the perisarc. The sexual organs are on the radial canals. The 

 medusae possess eye-spots (ocelli) and statocysts containing 

 statoliths of ectodermal origin. Examples: Obelia (Fig. 73), 

 Cam panid aria, Plumidaria, Sertularia, Clytia. 



Order 3. Trachymedusae. — Hydrozoa without alternation 

 of generations, the medusa developing more or less directly from 

 the egg. The sexual organs are on the radial canals. The 

 medusae possess sensory organs called tentaculocysts, contain- 

 ing- entodermal statoliths which are usually enclosed in vesicles. 

 Examples: Trachynema, Persa, and Liriope. 



Order 4. Narcomedusae. — Hydrozoa without alternation 

 of generations. The sexual organs are on the subumbral floor 

 of the gastric cavity or gastric pouches. The tentaculocysts 

 contain entodermal statoliths which are not enclosed in vesicles. 

 Examples: Cunocantha, Canina. 



