270 HYDROID^. Part IV. 



parent, to lead an independent life {Figs. 1 and 2), another set takes their place, 

 and then another after these, and so on successively (PI. XXIV. Figs. 19, 20, 21, 

 23, 24, and 22), till the whole of the germ-hasis is exhausted, and the proboscis 

 {Fig. 22, d) is left uncovered. As the germs leave the basis, they lose the 

 yellow tint of the latter, and become colorless. Their mode of development is 

 the same as we have described in Parypha (PL XXIII. Figs. 21, 22, 23, 24, 25, 

 and 26, p. 254), and therefore need not be repeated here. 



A short time before birth the young hydroid is endowed with a horny sheath, 

 like that of Parypha (PI. XXIII. Fig. 14^ c); but in Tubularia we have been able 

 to trace it, not only to the base of the head, but even to the tips of the tenta- 

 cles of the crown (PI. XXVI. Figs. 1 and 2, b) and of the proboscis (c). On 

 the stem of the hydra, it is as thick as in Parypha (XXIII. Fig. 14% c); but at 

 the base of the head it thins out suddenly, to a very thin, and yet distinct and 

 measurable, fihn, in which state it covers the whole head and tentacles.^ At birth 

 there are from ten (PI. II. Figs. 1 and 2, b) to fourteen tapering coronal tenta- 

 cles, and from eight to twelve proboscidal tentacles {c) ; the latter are mere 

 papillee, and constitute nothing more than a crenulate edge to the mouth. The 

 spermatic particles are similar to those of Parypha (PI. XXIII. Fig. 10). 



Proles medasoidea. — The mode of development of the medusa of Tubularia 

 Couthouyi is, with one single exception, identical with that of Thamnocnidia spec- 

 tabilis; we may, therefore, after pointing out the difference, refer the reader to 

 the embryology of the latter genus for farther details. In Thamnocnidia the inner 

 wall (PI. XXII. Figs. 1-8) rises as a solid layer, and, in time, forms a uniform 

 lining {Fig. 8", b) to the inner surface of the outer wall {Fig. 8% a), whereas, in 

 Tubularia Couthouyi, as the inner wall rises, it is channelled (PI. XXIV. Figs. 8, 

 9, 10, 11, 12, and 13) in the same way, as we have fully described in Coryne 

 mirabilis (PI. XVIII. Figs. 4-12, p. 192). In the last period of the breeding season 

 of Coryne, the male medusoids (PI. XVII. Fig. 11, n r) of this genus bear a strong 

 resemblance to the males (PL XXIV. Fig. 13) of Tubularia, and might easily be 

 mistaken one for the other. 



Histology. — All that we have to say of the histology of Tubularia has refer- 

 ence to the stem. The outer wall (PL XXIIP. Figs. 8 and 9, b) is about one 

 five hundredth of an inch thick, and consists of a mass of moderate sized, polyg- 

 onal cells, which are disposed in an irregular manner throughout the thickness of 

 the wall. On an average, they are about one four thousandth of an inch in 



* The alcoholic specimens of Tubularia indivisa sheath, to all appearances identical, both in pre- 

 sent to us by Sars, happen to be full of young, portions and extent, with that of our American 

 which, upon examination, we tind to possess a horny Tubularia Couthouyi. 



