92 



HYDROZOA. 



ceived in the centre of the tentacular zone, which becomes the proper 

 body of the polyp, or rather, forms the walls of its stomachal cavity. 



(214.) The walls of the bud which has hitherto contained the 

 embryo now become ruptured, and it gains its liberty (fig. 44, H). In 

 this condition it almost exactly resembles a young Hydra in its con- 

 tracted state ; and, in fact, both its body and its tentacula seem to have 



Fig. 44. 



Development of Tubularia from ova. 



the same anatomical structure as those of that simply organized polyp. 

 Having attained to this condition, its development proceeds rapidly, 

 and it soon begins to assume the specific form of the Tubularia from 

 which it sprung (fig. 44, i). 



(215.) SERTULABID^E. In the Sertularian Hydrozoa, the fleshy sub- 

 stance of the animal is enclosed in a ramose horny sheath, which it 

 traverses like the pith of a tree, following all the ramifications of the 

 branched stem of the polypary. 



(216.) Zoophytes of this description are readily found on our own 

 coasts, and the microscopic observer can scarcely enjoy a richer treat 

 than the examination of them affords. In order to study them satisfac- 

 torily, it is necessary to be provided with several glass troughs, of dif- 

 ferent depths, in which the living animals immersed in their native 

 element may be placed : in this situation, if the water be carefully re- 

 newed at short intervals, they will live for some time. 



(217.) Besides the cells which contain the polyps, others, specially 

 destined to the development of the ova, exist at certain periods of the 

 year ; they are larger than the preceding, and of a very different shape ; 

 but of these we shall have occasion to speak more fully hereafter. 



