x ELEUTHEROBLASTEA HYDRA 253 



projecting into the vesicle ; in other Trachomedusae, however, 

 the vesicle is open, but forms a hood for the protection of the 

 statorhab ; and in others, but especially in the younger stages of 

 development, the statorhab is not sunk into the margin of the 

 umbrella, and resembles a short but loaded tentacle. Eecent 

 researches have shown that there is a complete series of connect- 

 ing links between the vesiculate statocyst of the Leptomedusae 

 and the free tentaculate statorhab of the Trachomedusae, and 

 there can be little doubt of their general homology. 



In the free-swimming or " Phanerocodonic " medusome the 

 sexual cells are borne by the ectoderm of the sub-umbrellar 

 cavity either on the walls of the manubrium or subjacent to 

 the course of the radial canals. 



Order I. Eleutheroblastea. 



This order is constituted mainly for the well-known genus 

 Hydra. By some authors Hydra is regarded as an aberrant 

 member of the order Gymnoblastea, to which it is undoubtedly 

 in many respects allied, but it presents so many features of 

 special interest that it is better to keep it in a distinct group. 



Hydra is one of the few. examples of exclusively fresh-water 

 Coelenterates, and like so many of the smaller fresh-water 

 animals its distribution is almost cosmopolitan. It occurs not 

 only in Europe and North America, but in New Zealand, 

 Australia, tropical central Africa, and tropical central America. 



Hydra is found in this country in clear, still fresh water 

 attached to the stalks or leaves of weeds. When fully expanded 

 it may be 25 mm. in length, but when completely retracted the 

 same individual may be not more than 3 mm. long. The tubular 

 body-wall is built up of ectoderm and endoderm, enclosing a 

 simple undivided coelenteric cavity. The mouth is situated on 

 the summit of the conical hypostome, and at the base of this 

 there is a crown of long, delicate, but hollow tentacles. The 

 number of tentacles is usually six in H. vulgaris and H oligactis} 

 and eight in H. viridis, but it is variable in all species. 



During the greater part of the summer the number of indi- 

 viduals is rapidly increased by gemmation. The young Hydras 

 produced by gemmation are usually detached from their parents 



1 H. Jung, Morph. JaJirb. viii. 1881, p. 339. 



