DEVELOPMENT OF HYDROZOA 



869 



661) belonging to the sub-order Athecata. At A is sho^n the ali- 

 mentary zooid, or polypite, with its tentacles, and at B the succes- 

 sive stages a, b, c, of the sexual zooids, or medusa-buds. When 

 sufficiently developed the Medusa swims away, and as it grows to 

 maturity enlarges its manubrium, so that it hangs below the bell. 

 The Medusae of the genus Si/ncori/ne (as now restricted) have the 

 form named Sarsta in honour of the Swedish naturalist Sars. Theii 

 normal character is that of free swimmers ; but Agassiz ascertained 

 that in some cases towards the 

 end of the breeding season the 

 sexual zooids remain fixed, and 

 mature their products while at- 

 tached to the zoophyte. 1 This 

 latter condition of the sexual 

 zooids is very common amongst 

 the Hydroida ; and various inter- 

 mediate stages may be traced in 

 different genera, between the 

 mode in which the goiiozooids 

 are produced in the common 

 Hydra, as .-dread v described, and 

 that of Syncoryne. In Tubn- 

 laria the goiiozooids, though 

 permanently attached, are fur- 

 nished with swimming bells, 

 having four tubercles repre- 

 senting marginal tentacles. A 

 common and interesting species, 

 Tnbularia imHrixii., receives its 

 specific name from the infre- 

 quency with which branches are 

 given off from the stems, these f< >r 

 the most part standing erect and 

 parallel, like the stalks of corn, 



upon the base to which they are Fj(; 661 ._Development of Medusa-buds 



in Si/i/i'iii-i/iir Sa/rsii: A, an ordinary 

 polype, with its club-shaped body covrn :< I 

 with tentacles ; B, a polype putting forth 

 medusoid gemmae ; a, a very young bud ; 

 b, a bud more advanced, the quadran- 

 gular form of which, with the four 

 nuc-h'i whence the cirrhi afterward* 

 spring, is shown at d; c, a bud still 

 more advanced. 



zoo- 

 grows 



attached. This beautiful 

 phyte, which sometimes 

 between the tide-marks, but is 

 more abundantly obtained l>v 

 dredging in deep water, oft en 

 attains a size which renders it 

 scarcely a microscopic object. it> 

 stems being sometimes no less 

 than a foot in height and a line in diameter. Several curious 

 phenomena, however, are brought into view by microscopic examina- 

 tion. The polype-stomach is connected with the cavity of the 

 stem by a circular opening, which is surrounded by a sphincter ; 

 and an alternate movement of dilatation and contraction takes 

 place in it, fluid being apparently forced up from below, and then 

 expelled again, after which the sphincter closes in preparation for 



1 Hincks, op. cit. p. 49. 



