92 



CCELENTEEATA 



Fig. 148 



Hydra viridissima 

 (Stissw. F. Deut.). 

 1, sperm ; 2, ovum. 



ORDER 1. HYDRARIAE.* (THE HYDRAS.) 



Elongate, cylindrical animals 1 to 3 centimeters in length. The animal 

 attaches itself temporarily by means of a sticky secretion at one end 

 which may be called the foot, and can move about slowly by gliding on 

 the foot and by a looping movement; it cannot swim. At the free end 

 is the mouth on the conical hypostome, at the base of 

 which is a single row of hollow tentacles. No perisarc 

 is present. The food consists of small crustaceans 

 and other animals which are caught by means of the 

 nematoeysts. When the animal is well fed it repro- 

 duces principally by budding, the buds often remain- 

 ing attached to the parent for a while, but finally 

 becoming separated: it also reproduces occasionally 

 by transverse or longitudinal fission. At certain times 

 the Hydra reproduces sexually and is either monoe- 

 cious or unisexual. Spermatozoa are produced from 

 specialized interstitial cells usually near the base of the tentacles which 

 form rudimentary testes, and a single large ovum may appear in an ovary 

 formed of interstitial cells usually near the proximal end of the body. 



The order contains about four genera, of which Hydra 

 is the most familiar. The other genera, Protohydra and 

 Haleremita, which are marine, and Poly podium, which was 

 found on the eggs of the sturgeon in the river Volga, are 

 apparently rare and have not been found in this country. 

 HYDRA L. Tentacles 4 to 12 in number: 3 well 

 established species; cosmopolitan; in fresh water. 



H. viridissima Pallas (H. viridis L.) (Fig. 148). 

 Green hydra. Body grass green, the color being due to the 

 presence of zoochlorellae in the entoderm; tentacles short 

 and about 6 in number; hermaphroditic: usually on water 

 plants. 



H. oligactis Pallas (H. fusca L.) (Fig. 149). Brown 

 hydra. Body brown; tentacles very long and about 8 in 

 number ; proximal end of body slender and stalk-like ; unisexual : on stones, 

 sticks, and plants. 



H. vulgaris Pallas (H. grisea L.). Color gray, orange, or brown; 

 proximal end of body not stalk-like; average number of tentacles 6: on 

 stones, plants, etc. 



* See "M6m. pour servir a 1'Hist. d'un genre de Polyps d'eau douce," by A. 

 Trembley, 1744. "The Development of Hydra," by G. A. Tannreuther, Biol. Bull., Vol. 

 14, p. 261, 1908. "Die Benennung und Unterscheidung der Hydra Arten," by 

 A. Brauer, Zool. Anz., Vol. 33, p. 790, 1909. 



Fig. 149 



Hydra oligac- 

 tis (Stissw. F. 

 Deut.). 



