Chap. IV. PARYPHA CROCEA. 



259 



highest part, which is nearest the aperture of the disk, assumes an arched contour 

 {Fig. 10, /). In consequence of the mode of formation of the tentacles, their 

 interior is hollow. If we look along the plane of one (through a-c, Fig. 12^), 

 a very narrow cavity {Fig. 12^ b) may be seen, between the opposite double walls' 

 In profile the double walls {Fig. 12% d e) are very readily detected; the outer one 

 {d) is very thin, but the inner one {e) is extraordinarily thick, next the top of the 

 crest.i ^ Along the whole length of the base of the crest a narrow fissure {Fig. 12% 

 b b' Ir) extends, and serves as a passage-way from the cavity of the tentacle 

 into the interior of the disk. When the young hydroids are pretty far advanced 

 in their development, and are numerous, the medusoid parent becomes spherical 

 {Fig. 12j and the tentacles (/) higher in proportion to their length, and the transi- 

 tion into the disk is not so gradual. When most of the young have escaped 

 from the parent, the latter becomes elongate {Fig. 14), and the tentacles {f f) 

 assume a great height, equal to two thirds the length of the base, which now 

 occupies much less extent. In this condition, when seen with a low magnifying 

 power, they appear like cylindrical bodies {Fig. V, b), and are very conrpicuout 

 among the other, globose or oval, medusoids. The proboscis, during these later 

 periods, moves about with great freedom, and oftentimes projects far beyond the 

 aperture of the disk, but as the medusoid approaches the end of its breeding 

 season, and the young are nearly all discharged, the proboscis diminishes and 

 retracts toward its base {Fig. 14, d), but yet retains its activity. When the 

 germ-basis has almost separated from the proboscis, the part which remains {Fig. 17, a) 

 oftentimes appears like an outer wall, but in later stages {Fig. 14, d), when the 

 germ-basis is altogether developed, the proboscis is clearly single-walled. 



Throughout the whole course of the development of the medusoid, thefe has 

 not been the least trace of radiating or circular chymiferous tubes within the 

 disk; and the only place where chyme has been seen to circulate was in the 

 proboscis, which we have shown above to be in open communication with the 

 canal of the pedicel. After the young are all excluded, the parent shrivels, dies, 

 and falls off The medusa-buds of Parypha thus appear to belong to the simplest 

 forms of Acalephs. Morphologically, they are as genuine Medusje as those which, 

 being freed from their parent stock, assume an independent life, during which the' 

 reproductive organs are developed at a late stage of their life; but, as far as 

 the complication of this structure goes, they do not rise above the level of ordinary 

 Hydroids, being, like them, destitute of radiating and circular chymiferous tubes. 



' These two walls are so conspicuous, when genus has been called Parypha, from TTaovg,,;, a 

 seen in profile, as to give the tentacles the appear- border or hem. Another species of this genus has 

 ance of being margined, and, on this account, the been described by McCrady as Tubularia cristaia. 



