310 



HYDROID^. 



Part IV. 



Fhj. 46. 



Quarter segment of a very young TiA- 



ROPSIS. 



ab (I secondary tentacles. — e e primary tentacles. 

 — g chymiferous tubes. — c compound eyes. — 

 f opening of the veil. 



base, as in the latter, had a Large cavity communicating with the circular tube. 

 On both sides of, and in immediate juxtaposition to, 

 each eye {c), there was a tentacle {h) nearly as far 

 advanced as the secondary ones («), but the base 

 was as yet only slightly swollen, and contained 

 scarcely an}' pigment cells within its cavity. Of these, 

 the third group, there were sixteen, two for each of 

 the eight compound eyes (c). The fourth, and young- 

 est group {d), amounted to sixteen, one on each side 

 of every primary {e) and secondary (a) tentacle. 

 They were scarcely more than one third as long as 

 those of the third group, and had perfectly trans- 

 parent bases, which, however, Avere very nearly as 

 broad as the bases of the third group. All the ten- 

 tacles of the first, second, and third groups, bristled 

 with well-developed lasso-cells, and, in the fourth group, 



these bodies were in a rudimentary state, just far enough advanced to appear like 

 minute specks in the walls of the tentacles. The highly refractive corpuscules of 

 the eyes (c) numbered no less than six or seven in each eye, and were arranged 

 parallel-wise to the edge of the disk. There was also a pigment spot at the base 

 of each eye, which was already so dense as to be more conspicuous than the 

 refractive corpuscules. It thus appears that, after the first, the others, secon- 

 dary tentacles, follow regularly in pairs. The mode of development of tentacles 

 is very simple, and may be comprehended at a glance by inspecting the figures 

 which we have given (PI. XXXI. Ficjs. 10 and 11), to illustrate this process. The 

 outer wall {Fig. 10, a}) of the edge of the disk, together with the inner one (6'), 

 protrude in the form of a double-walled papilla {a b); this papilla continues to 

 grow for a while by the same process with which it commenced; and in this 

 way a hollow, double-walled {Fig. 11, a b), broad cone is produced. From this 



hollow base the solid portion, or tentacle proper, is devel- 

 oped ; but we have not traced its cellular growth, and 

 therefore cannot point out any thing beyond the general 

 increase in proportions, size, and appearance, as we have 

 done above for the medusa with forty tentacles. As the 

 animal increases in size, the bell gradually broadens, as may 

 be seen in our figures of a specimen one eighth of an 

 inch in diameter (wood-cut 47). It has fifty-two tentacles, 

 twelve between every two of the primary ones; the upper, 

 or abactinal half {b) of the disk, is still as high as one fifth the transverse diam- 



Fig. 47. 



Young TiAROPSis with fifty- 

 two tentacles, and magnified 

 disk. 



