274 HYDROID^. Pakt IV. 



(p) more sharply conical, and the tentacles (te) at least three times as long as the 

 transverse diameter of the base of the head, and slightly swollen at the tips. In 

 this condition it escapes from the parent, and after creeping about for a while, 

 settles down upon its stem, expands its tentacles {Fiff. 15, t) and its hitherto 

 unseen mouth {f), and five, six, or seven buccal tentacles (f). The number 

 of discal tentacles, at the birth of the young hydroid, varies from seven or eight 

 to eleven. At first, they are rather crooked and rough {J^iff- 15, t), but very 

 soon they assume the smooth contour of the adult, but retain their four-sided 

 shape, as described in the young of Parypha (p. 254). In regard to the walls of 

 the body and the tentacles, the details of the mode of escape from the parent, 

 and the appearance of the last portion of the germ-basis, we may refer to Parypha 

 crocea. There is a wide difference between the degree of development to which 

 the several embryos, in one and the same parent, have arrived ; one of them 

 {Fiff. 14, st) may be just escaping from the medusoid, whilst another is a mere 

 irregular, spherical mass (e'), without any traces of organs, and yet there still 

 remains, clinging to the proboscis, enough of the germ-basis {e) to form a third 

 individual. 



Proles medusoidca. — We have already pointed out, on the preceding page, the 

 identity in the mode of development of the earliest stages of the medusoids of 

 Parypha and Thamnocnidia, and, therefore, need not repeat these descrijitions here. 

 After the inner wall (PI. XXII. Fig. 2, b h^) has become deeply cup-shaped, there 

 arises a difference between these two genera, in their mode of growth. In Parypha 

 the proboscis (PI. XXIII. Fig, 6, d) arises from the base of the cup, before the 

 edge of the latter has reached the extreme of the bud ; whereas, in Thamnoc- 

 nidia, the edge of the cup, having followed the inner surface of the exterior wall 

 (PI. XXII. Fig. 3, a), and finally arching over and uniting its constricting lip, has 

 formed a continuous inner wall {b), as soon as the proboscis begins to rise in the 

 guise of a broad, low papilla ((/). In this way the germ-basis (e) is withdrawn 

 from contact with the outer wall (a) and shut up within the interior wall {b) 

 and its continuity, the single transverse wall [d) of the proboscis. Soon after 

 this the medusoid begins to broaden [Figs. 4 and 4*), and assumes a globular shape, 

 and the proboscis (c?) gradually pushes its way into the mass of the germ-basis 

 (e), while the latter, at the same rate, assumes a deeper and deeper concavo- 

 convex form, and becomes a cap to the former. Up to this time, the wall of the 

 proboscis {d) has maintained a pretty uniform thickness, about equal to that of 

 the inner wall {b) ; but subsequently it shows considerable variation in this respect, 

 probably owing to the different degrees of contraction in which it may be at 

 various times. Sometimes the wall {Fig. 5, d) swells till its cavity {c) is nearly 

 obliterated, and soon, again, it extends its peripheric dimensions at the exj^ense of 



