22 THE VOYAGE OF H.M.S. CHALLENGER. 



slender than in the gastrozooids, and a spheroidal tip resembling that of the tentacle of 

 the gastrozooid, but smaller. The body of the dactylozooid terminates sometimes in two, 

 sometimes in three tentacles, springing from a common point. 



The dactylozooids expand far more readily and tpiickly than the gastrozooids, of which 

 latter it is comparatively difficult to obtain a view in the expanded condition. The short 

 gastrozooids apjiear to remain perfectly quiescent when expanded, whdst the dactylozooids 

 are in constant serpentine motion. The dactylozooids seldom carry their bodies extended 

 straight, but usually bent in several curves ; they appear to bend over towards their 

 gastrozooid from time to time, as if to convey food. All the zooids are retracted on 

 alarm with remarkable suddenness, disappearing entirely within the pores. 



When a portion of the coral has been placed living in reagents, it is found, after 

 becoming hardened, to be bristling all over with sheaves of threads shot from the 

 nematocyst around the mouths of the calicles. By some accident, on one small portion 

 of a coral placed in absolute alcohol, the dactylozooids all remained partially protruded. 

 This was only over a small area of about J of a square inch in dimensions, enough to 

 yield a single microscopical preparation. From a very large quantity of the coral pre- 

 pared in an exactly similar manner, no second preparation could be obtained, though it 

 was all searched over carefully for similarly expanded zooids. This fact, however, shows 

 that perhaps it might have been possible to obtain a larger quantity of expanded zooids 

 in the hardened condition by the gradual addition of alcohol or fresh water to the sea- 

 water in which the living animals were expanded, or by some similar means ; or perhaps 

 by the sudden addition of osmic acid solution as recommended by F. E. Schulze. 1 



The body of the zooids, when seen in transverse section, is found to consist (PL XIV. 

 fig. 7) of an ectodermal layer, beneath which is a layer of membrane, and an internal 

 mass of endodermal cells. The ectodermal layer, as studied in sections of hardened 

 specimens, appears to consist of well-defined cells, most of which contain small nenia- 

 tocysts, whilst some contain simple nuclei. The membranous layer is apparently structure- 

 less ; it extends throughout the body and tentacles. Beneath the membranous layer, and 

 in close union with it, are the muscular structures to be presently described, and within 

 these, in the case of the gastrozooids, are, in the upper region of the body, the gastric 

 cells already described. The structure of the endoderm in the lower part of the body of 

 the gastrozooids, and in the dactylozooids, was not well ascertained. In transverse 

 hardened sections the body-cavity is seen to be entirely filled with the pigmented yellow 

 cells, which also fill the canals of the ccenosarc. In the tentacles of the dactylozooids, 

 however, of which a glance was obtained under a high power, the transverse lines or 

 apparent septa, so characteristic of the Hydroids (PI. XIV. fig. 5), and considered by 

 Allman to be in reality the opposed walls of large adjacent endodermal cells, were clearly 



1 Anleitung zu wissenschaftlichen Beobachtungen auf Reisen. Herausgegeben von C. Nemnayer, Hydrograph der 

 kaiserlichen Achniralitat, Berlin, 1875, Wirbellose Seethiere von K. Mobius, p. 424. 



