202 



HYDROID^. 



Part IV. 



Fig. 25. 



From a speci- 

 men of the same 

 size and age as 

 that of wood-cut 

 20.1 



Fig. 26. 



Fig. 27. 



the eighth of May the adult (wood-cut 30, p. 212) occurred in great numbers. On 

 the 17th of May the males and females contained, severally, abundance of sper- 

 matozoa and eggs. 



As we have said before, the outer and middle walls become widely separated, 

 at birth, but are nearly parallel to each other at first (PI. XVIII. 

 Fiffs. 15* and 17, wood-cut 25). Soon, however, they begin to assume 

 very different outlines; the outer one becoming more rounded and 

 expanded above, and the middle one more open below, so that the 

 two seem to approximate near the edge of the disk, and grad- 

 ually recede, going upward (wood-cut 26). This disparity increases 

 till the outer wall becomes oval in outline, and the middle one 

 conical (wood-cut 27). The outer wall, in this histance, is more elon. 

 gate-oval than in the adult, so that the disk is much higher than it is broad. 

 After this the disk grows proportionally broader (wood-cut 28), and the top of the 

 dome less pointed, till it has reached the 

 adult state (wood-cut 30). The aperture 

 (wood-cuts 25, 26, and 27, c) in the trans- 

 verse partition gradually increases in diam- 

 eter with the growth of the disk, till, by 

 the time the latter is one fifth of an inch 

 in diameter, it is as large, in proportion to 

 the size of the animal, as in the adult. 

 Specimens of this Medusa which have reached two thirds of 



their normal size (wood-cut 28, p. 211), are capable of stretching their tentacles to 

 as great a length as the adults. The adult is not only able to contract into a 

 very small compass, but also to stretch longitudinally at the expense of its breadth, 

 till it is twice as long as broad (wood-cut 31, p. 212). While doing this the 

 transverse partition (c^) is oftentimes allowed to hang down loosely, in an inverted 

 truncate-conical shape. The extent to Avliich the proboscis may contract and expand 

 may be inferred from a comparison of the two figures, wood-cuts 29, d, and 31, d, 

 p. 212 ; in the first, it is stretched to four times the length of the disk, and con- 

 siderably expanded withal, and in the second retracted so as hardly to equal one 

 half the height of the disk in a quiescent state. The tentacles, at times, remind 

 one of the long cirrhate arms of Pleurobrachia, when, instead of stretching uniformly. 



^ Wood-cuts 25, 2G, and 27 represent the suc- 

 cessive changes which take place in the shape of 

 the disk as the medusa develops after being freed, 

 magnified 25 diameters. a indicates the outer 



surface, and b tlie inner surface, of the disk ; a^ 

 the depression in the top of the disk ; b^ the 

 thickening of the centre of the disk ; c the aper- 

 ture of the veil. 



