Chap. IV. THAMNOCNIDIA SPECTABILIS. 275 



its thickness {Fig. 6, d). After this, the inner wall {Fir/. 8% b) of the disk, which 

 hitherto has maintained a thickness equal to that of the outer wall, ceases to 

 grow so rapidly as the latter {Fig. 8% a), and, about this time, the walls of the 

 area, around the future aperture of the disk, rise in the form of papillae {Figs. 8 

 and 8% /), varying from two to four in number. The papillae are, homologically, 

 the tentacles, although they do not ever seem to perform the office of such organs, 

 even when most fully developed. In some medusoids unmistakable signs of a 

 greater age than this may be discovered, and yet the tentacles have not begun 

 to develop. Thus, in certain individuals, the germ-basis {Fig. 7, e) has changed 

 to the characteristic yellow color of the later stages, but there are no tentacular 

 appendages on the disk. The individual figured is an instance of the plasticity 

 of the medusoid, which, at times, may be seen very much elongated, and then, 

 again, concentrated upon itself in a globular form. The subsequent development 

 of the medusoid consists in the elongation of the tentacles {Figs. 9-14, /) and the 

 diminution of the thickness of the inner Avail of the disk, imtil it appears like 

 a mere filmy epidermis upon the interior of the outer wall, and, unless highly 

 magnified, cannot be seen. On this account the disk appears to have only a 

 single wall in the figures {Figs. 9-14) representing the later and last phases of 

 growth. Owing to the dense red pigment granules, which collect in large numbers 

 along the sides, and especially at the tip {Figs. 8, 9, and 10, d) of the chymiferous 

 cavity of the proboscis, the whole medusoid is pervaded by a delicate pink tint, 

 which, when seen in a certain light, combines with the yellow color of the germ- 

 basis to form an orange hue. The mouth of the disk is formed very late, prob- 

 ably not until the young is just ready to leave the parent. 



Thamnocnidia tenella Ag. Proles hjdroidea. Adult. — Although this species agrees 

 so closely, in nearly all its details, with T. spectabilis, it has a very different 

 habitat; it is never found, with its congener, in brackish water, but always in the 

 open ocean, among rocky pools. It is a very delicate, graceful animal, and much 

 the smallest of our Tubularians, having about half the size of T. spectabilis or 

 Parypha crocea. It branches very irregularly, loosely, and openly (PI. XXII. 

 Fig. 21), with a stem of uniform thickness throughout, about as large as a com- 

 mon sewing needle, or, to be more exact, one fiftieth of an inch in diameter. 

 The medusoids have been observed in January, July, August, and December, but 

 the young hydroids were only seen escaping from' the parent during the months 

 of July and August. 



VanBeneden has given a very incorrect accoimt of the reproduction of the 

 Tubularians, in his paper on the Embryology of these Hydroids. According to 

 his representation, the medusoids, after freeing themselves from their parent stock, 

 attach themselves to submarine bodies, and grow up into new hydroids. This 



