Chap. IV. THAMNOCNIDIA SPECTABILIS. 273 



and the fluid contents of the stomach circulate very rapidly, a phenomenon which, 

 owing to the transparency of the distended walls, may be seen very readily. As 

 an opposite extreme to this, the proboscis stretches out till it equals in length 

 the greatest extension of the discal tentacles {Fig. 16, ^>'), and is as slender as 

 the thickest part of the stem. 



Proles medusoidea. — The full-grown medusoid is a very simple, double-walled body 

 (PI. XXII. Ficf. 14), with a thick proboscis (e) projecting half way or less into its 

 cavity, while at the opposite end, around the lower edge of the disk, are three or 

 four solid, short, and rather unshapely tentacles (/). Excepting the tentacles, the 

 whole structure of the medusoid is almost identical with that of Parypha crocea, 

 even to the absence of radiating and circular chymiferous tubes. As it produces 

 only two or three young, it is seldom distended in a lateral direction, as in the 

 latter genus, but usually presents an elongated form {Figs. 13 and 14), produced 

 by the efforts of the young to push their way out through the actinal opening 

 of the disk. 



Emhryology. Proles hjdroidea. — The description of the development of the young 

 hydroids of Parypha crocea, up to the time when the tentacles have assumed an 

 oblong shape (PL XXIII. Fig. 13, e^), applies perfectly well to Thamnocnidia, and 

 therefore needs not be repeated here. Beyond this, however, there are certain differ- 

 ences, not so much of structure as of form, in their relation to the parent body, which 

 require special notice. Owing to the small number of the embryos, there being only 

 two or three produced by each medusoid, and to the fact that they are developed 

 consecutively, each young hydroid (PI. XXII. Fig. 12, si) occupies a large proportion 

 of the cavity of the disk. There is, however, one remarkable feature in regard to 

 the position of the young in the parent, which at once distinguishes this genus from 

 Parypha, namely, that from a very early period the young is frequently, if not 

 always, fitted like a cap over the end of the proboscis {Fig. 12, d d^), or rather over 

 the germ-basis {e) which covers the proboscis. In this position the base {st) of the 

 young hydroid occupies the region near the opening of the disk of the parent 

 medusoid, and the tentacles {te) embrace the base of the proboscis {d). From this 

 time forward, the development of the embryo is very rapid. The mass of the body 

 forming the cap to the proboscis, becomes proportionally smaller {Fig. 13, st) and 

 constricted just behind the base of the tentacles {ic). This part constitutes the 

 stem. The proboscis first appears in the form of a broad, conical protuberance {te), 

 within the circle of tentacles, while the latter become simply elongate and tapering 

 to a blunt end. In the mean while the proboscis of the medusoid, with its covering, 

 the germ-basis {c), retracts from, or rather is pushed out of its cap-like socket by the 

 protruding proboscis of the hydroid. After this the embryo seems actuallv to grow 

 larger, while the stem {Fig. 14, st) becomes a little longer than broad, the proboscis 



vol.. IV. 35 



