116 



HYDKOZOA. 



gether gelatinous ; its tentacles are movable in all directions and ex- 

 ceedingly irritable, and its whole structure and appearance, in short, 

 that of a gelatinous polyp or Hydra. But a great change is now in pre- 

 paration : the body of the Hydriform polyp gradually increases in size ; 

 and transverse folds begin to make their appearance at equal distances, 

 one below the other, partitioning off its body into numerous rings or 

 segments (fig. 54, 15). 



Fig. 54. 



Development of fyanea capillata (after Sara, ' Annalee des Sciences Naturelles ' for 1841, plates 

 15 B, 16, & 17, pp. 19, 50). 1. Young Acalephs newly hatched (natural size). 2. One magnified, 

 showing infusorial condition of development. 3, 4, 5, 6. The same animal now become attached 

 by a pedicle, and gradually assuming the polypoid form. 7. A still more advanced condition, 

 showing the mouth surrounded by numerous retracted tentacula : the mouth is dilated, exhibit- 

 ing four longitudinal eminences, situated in the stomachal cavity. 8. The same individual cut open 

 longitudinally, and spread out so as to show the longitudinal eminences in the interior : the 

 transverse lines are caused by the contraction of the body. 9, 10. Two polypoid Acalephs, with 

 stolons developed from the upper part of the body : in fig. 10 the stolon has become attached to the 

 supporting surface. 11. Fully-developed polyp. 12. Another individual giving off a stolon, from 

 which proceeds a second that in like manner gives off a third offset. 13. Stolons growing off from 

 the base of the polypoid Medusa, which, creeping along the surface of the substance to which it is 

 attached, give origin to new polyps, a, b. 14. Three young gemmae sprouting from the body of a 

 polypoid Acaleph. 15. A polypoid larva magnified (the natural size is shown at 15 a), having its 

 body divided by numerous transverse wrinkles. 



(303.) In the course of a short time the segments thus formed be- 

 come surrounded with marginal rays dichotomously divided at their 

 extremities. These rays or arms are free, having their apices directed 

 upwards, and disposed with such regularity that the once polypoid body 

 seems to be furnished with eight longitudinal ribs (fig. 55, 16). 



(304.) "We now arrive at the fourth period of the process, when the 

 different segments into which the original polyp has become divided 

 separate from each other, so as to form so many distinct disks (Planulce, 

 Dalyell), each of which on its separation becomes a complete animal. 

 This separation commences at the upper extremity of the series of 

 newly-formed beings, and is repeated, segment after segment, towards 



