HISTOLOGY OF THE ECTODERM. 115 



The muscular sac which thus lines the umbrella in Syncoryne pidehella is not uniformly 

 adherent to it, and receives its chief attachment along eight meridional lines. Four of these cor- 

 respond to the direction of the four radiating canals, and the remaining four arc so distributed 

 that one lies exactly midway between every two radiating canals. 



In medusae, such especially as have been kept alive for some time in our jars, the muscular 

 sac may be occasionally seen to be separated by a considerable interval from the rest of the 

 umbrella in all the spaces which intervene between the eight meridional lines of attachment, 

 tiiough it still continues closely adherent to these lines (woodcut, fig. 49 a). A few delicate 

 bands may here and there be seen, near the summit of the umbrella, stretching transversely across 

 the spaces betw'cen the umbrella and the detached portions of the sac. 



It is in this condition that the inner epithelium layer becomes apparent. I have failed to see 

 it as long as the muscle-sac is uniformly in contact with the rest of the umbrella. 



An inner epithelium layer has been shown by Haeckel to exist in the Geryonidm. There, 

 however, he describes it as lying on the concave surface of the nuiscular layer. 



In the medusa of Obelia geniculata shortly after liberation, the inner epithelium is very 

 distinct, but I have failed in satisfying myself of the existence of a distinct nmscular layer in the 

 almost disc-shaped umbrella of this medusa, which, moreover, presents the very exceptional 

 condition of being entirely destitute of a velum. On the other hand, a distinctly fibrillated 

 layer may be seen in the marginal tentacles (woodcut, fig. 59 a a), which indeed would seem by 

 their fin-like action to be far more efficient than the umbrella in the locomotion of the medusa. 



The inner epithelium consists here of distinctly nucleated cells with narrow intercellular spaces. 

 In many of the cells the nuclei were plainly seen to be in process of division (woodcut, fig. 59 c). 



The gelatinous portion of the umbrella thus lies between the two layers of epithelium, and is 

 probably a product of one or both of these layers. In many hydroids the medusa immediately 

 after liberation has this gelatinous portion still thin, but it often acquires great thickness as the 

 medusa advances towards maturity — a phenomenon of which Bouguinvillia affords a striking 

 example (see PI. IX).^ 



NematojjJiores. — The general form and relations of the nematophores have been already 

 described (p. 28). The matter which fills the chitinous sheath of the nematophore is a clear semi- 

 fluid substance with scattered granules, and without the slightest trace of structure, but having 

 frequently imbedded in it a cluster of thread-cells. It differs in no respect from the sarcode 

 matter composing the bodies of the Rldzopoda, and like it is capable of emitting true pseudopodia. 

 When a specimen of Antenmilaria antennina (woodcut, fig. 50) or Aglaophenia pluma (woodcut, 

 fig. 51 J is examined in the zoophyte trough of the microscope shortly after removal from the sea, 

 and before it has lost any of its original vigour, the contents of the nematophore-sheaths will be 



^ Haeckel (op. cit.) has described in the umbrella of the Geryonidce certain structures which he has 

 shown to be nearly allied to cartilage, not onh' in consistence, but in histological composition. This 

 medusa-cartilage forms — 1. A ring which runs in the gelatinous substance of the umbrella parallel to 

 its margin, and below the circular canal. 2. Certain rib-like structures (mantel-spangen) which are 

 imbedded in the outer surface of the umbrella, and extend from the margin in a meridional direction 

 for a greater or less distance towards the summit. 3. The rod-like axis of the stiff, solid tentacles. 



In all these cases the medusa-cartilage consists of rouuded, nucleated, occasionally vacuolated, 

 masses of granular protoplasm, contained in cavities of a clear homogeneous iutercellular substance 

 having a cartilaginous consistence. 



