THE COMMON HYDRA OF BENGAL. 341 



spines in front of the barbs, and closely resembles that found in H. grisea. 1 '' A form 

 without these spines and smaller than the other occurs occasionally, being intermediate 

 between the largest and most complex and the simplest type, which is, as is the case 

 in other species, devoid of barbs. 



The smallest nematocysts are very much more numerous than the largest ones, 

 and as a rule each large cell is surrounded by small ones. Their arrangement is not, 

 however, by any means regular. Both these forms are more abundant on the upper 

 half of the body than on the lower. Their distribution has no definite relation to the 

 raised reticulation on the surface of the body. On the tentacles there are about four 

 large cells in each tract, but the number is not constant. In the terminal bead there is 

 a battery of the largest cells. 



The gonads will be described under the heading ' ' Reproduction." 



Dimensions — 



When fully expanded the body may be nearly 3 cm. long, but 1*5 cm. is a more 

 usual length. When the body is very much elongated, the tentacles are never fully 

 extended. 



The Species of Hydra. 



The attempts that have been made to ascertain the number of species and the best 

 diagnostic characters in the genus Hydra have been numerous. There can be no doubt 

 that the earlier authors of the eighteenth and nineteenth centuries described a number 

 of temporary phases as distinct forms. Such was the case, for instance, with Pallas' s 

 H. oligactis, the most important character of which, viz., the wine-glass shape of the 

 body, depended on its having recently fed. (See the figures given by Hincks, Brit. 

 Hydr. Zooph. I, p. 315, 1868.) In a similar manner colour was made a criterion, al- 

 though in some forms the colour of the individual changes from time to time. Even 

 in recent years different authors have regarded different characters as of specific impor- 

 tance. In 1882 Haacke 2 recognized three species ; Nussbaum recognized four in 1887, 

 as did Brauer 4 in 1891 ; but Brauer's four were not the same as Nussbaum's. Hargitt, 5 

 in 1901 only recognized two, but did not give diagnoses. Downing," in 1905, agreed with 

 Brauer, naming a form, H. dicecia, which the latter had recognized but not named. 

 Brauer and Downing' s classification, being founded on a combination of several charac- 

 ters, appears to be sound, provided that no attempt is made to identify individuals with- 

 out a study of their life-history. It must be frankly confessed that in many instances 

 the only distinctive characters which could be noticed with certainty, if even they 

 were recognizable, would be those on which Hargitt probably relied. This, however, is 

 only true in the case of individuals which are not sexually mature, and of males. 



Leaving out of consideration such doubtful forms asAsper's 7 H. rhcetica from 



1 See Nussbaum in Archiv f. micr. Anat., XXIX, 1887, pi. XIII, fig. 10. 



2 Jena. Zeitschr.f. Med. u. Natur-w , XIV, 1880, p. 134. 5 Amer. Naturalist, XXXV, 1901, p. 301. 



3 Archiv. f. micr. Anat., XXIX, 1887, p. 272 6 Zool. Jahrb., Anat., XXI, 3, 1905, p. 381. 

 * Zeitsckr. f. miss. Zool., LII, 1891, p. 177. 1 Zool. Anz., 1880, p. 205. 



