102 HYDROZOA. 



ferior surface, there is developed a tubercle, destined hereafter to 

 become the body of the polyp ; it is by this part that it will ulti- 

 mately become attached. 



(255.) The four cells formed between the individual groups above 

 mentioned are, in this stage of the growth of the little being, so com- 

 pressed that they seem to be quite lost ; soon, however, they expand so 

 as to press upon their neighbours, and then the disk appears to be 

 surrounded with a regular series of cellules, twenty-four in number, 

 which, as they become developed, shoot out externally, and soon present 

 themselves under the appearance of so many tubercles (fig. 48, E). 



(256.) The eight interior cells (c c) take another direction, but their 

 form remains unchanged; and they exhibit, up to the termination of this 

 embryo condition of the animal, a nucleus in the centre of each, which 

 might be regarded either as a crystalline lens or an otolith, according 

 as these organs are judged to be eyes or auditory capsules ; for such are 

 the designations applied to them by modern zootornists, as will be ex- 

 plained in the next chapter. 



(257.) The marginal tubercles situated around the disk (fig. 48, E) now 

 become sensibly elongated, and the whole embryo presents the appear- 

 ance of a minute star-fish, the elongate tubercles representing the rays. 



(258.) The nuclei in the interior of the marginal (tentacular) 

 tubercles next become elongated, together with their containing cells, 

 rendering the rays hollow in the centre ; and soon new cells are dis- 

 coverable in their interior, the number of which is limited, and probably 

 the same, in all the rays. The appearance of these secondary cells 

 causes a rapid increase in the length of the tentacula, and their remains 

 give rise to numerous septa, producing an appearance somewhat ana- 

 logous to that of the transverse striae of muscular fibre. 



(259.) The embryo animal, be it observed, is as yet still contained in 

 the ovary of the polyp; but it is already capable of distinct and con- 

 tinual movements, perceptible through the walls of the ovarian vesicle. 



(260.) A tubercle (fig. 48, F, 6) is developed from the centre of the 

 under surface of the disk, which represents the central pedicle met 

 with under various forms among the Medusae, and which may be called 

 the proboscidiform appendage. This organ can contract or extend itself 

 in all directions, constantly changing its form, and resembling in no 

 slight degree the body of a Hydra. At an early period an opening is 

 perceptible at the extremity of this appendage, which evidently repre- 

 sents a mouth, being in direct communication with the vitelline cavity. 



(261.) The vitelline or digestive cavity, now that there is a mouth, 

 increases in size in proportion to the growth of the embryo, but still 

 preserves its sac-like shape. It is partially filled up with irregular 

 granules, which become perceptible at a very early period at first 

 colourless, but gradually becoming of a yellowish tinge. Towards the 

 close of this period of development the granules seem to be heaped 



