194 



HYDROID^. 



Part IV. 



and wood-cut 11, c c' c^) have become twice as long as they are broad, and, in 

 addition to this, a new feature is introduced, in the form of a broad and short 



hernia (wood-cut 11, h), which arises from the bottom of the 

 cup. The four channelled lobes, or radiating chymiferous 

 tubes, as they are designated in the full-grown medusa, press 

 closely against the hernia. From the relation which this 

 hernia bears to the radiating tubes, and its position in the 

 bud, it is evident that it is the proboscis of the growing 

 medusa, and as such we will hereafter designate it, even 

 though it is not yet Open, as in the adult. The chymiferous 

 fluid circulates freely in the proboscis, and may be seen, 

 at various times, whirling to and fro, in gyratory currents, 

 with greater or lesser velocity. 



On account of the position of the medusoids, it is not easy to obtain a distinct 

 end view of them, except now and then, when they are situated on the neck of 

 the hydroid, which is not so dark as the head. In this position we have observed 

 a medusoid, hardly older than the one just partly described, which may very well 

 serve to illustrate the peculiar relations Avhich the radiating tubes bear to each 

 other and to the proboscis. These tubes (PI. XVIII. Fir/. 8", 

 and wood-cut 12, c) are a great deal bi'oader than at the time 

 they were formed (wood-cut 10, c) ; they are, in fact, so much 

 expanded that they touch each other at their extreme edges 

 (wood-cut 12, k). In consequence of this, the single wall 

 (wood-cut 10, iw), which was quite conspicuous between the chy- 

 miferous tubes of the earliest stages, is here almost evanescent. 

 In a transversely sectional view, the ch^'miferous tubes are semi- 

 cylindrical, and have the flat side {Fiff. 8" and wood-cut 12, ?«') next to the pro- 

 boscis (to). The channels of these tubes are also segments of a cylinder. The 

 rectangular disposition of these tubes corresponds with the shape of the proboscis, 

 which has a square outline, with sides (m) running parallel to the faces of the 

 tubes. Its cavity {h), however, does not accord with the contour of the wall, but 

 is perfectly circular in outline. The space not occupied by the proboscis is still 

 tilled by the thickening of the outer wall (PI. XVIII. Fi(/. 8, and wood-cut 11, d ; 

 Fig. 8", and wood-cut 12, m m^). Presently, however, this thickened part becomes 

 hollowed, at the region opposite the proboscis, to such an extent, that only a mod- 

 erately thick laj^er (PI. XVIII. Fig. 9, b^ n, and wood-cut 13, b^ n) is left as a lining 

 to the cup {a) formed by the inner wall and its hernia, the proboscis («^). An 

 ideal vertical section through the medusiB-bud, cutting the walls at two opposite 

 points, between the radiating tubes, may lead to a clearer view of the relations 



Fig. 12. 



