HISTOLOGY OF THE ECTODERM. Ill 



Sometimes, however, the cctoderiu of the adult hydroid may be seen to be composed of 

 very distinct cells, which can occasionally be isolated without difficulty under the microscope. 

 In Hydra viridis, for example, the ectoderm can be broken down under the microscope into 

 spherical cells, in which a nucleolated nucleus may occasionally be detected. These cells are 

 provided with distinct membranous walls, within which is contained a homogeneous vacuolated 

 protoplasm, in which are often immersed secondary nucleated cells, and also in many of them one 

 or more thread-cells. 



In some cases in which it is not easy to detect definite structure in the normal state of the 

 ectoderm, this membrane, by a natural hystolytic decomposition after death, will become broken 

 up into very distinct cells (PI. I, fig. 4). 



Faljmcils. — Leydig^ has called attention to the occurrence in Ili/dra of a minute bristle-like 

 projection of the surface over the site of each thread-cell, without having any immediate connection 

 with the thread-cell itself, and Dujardin" had already noticed and correctly described similar 

 bodies as existing on the surface of the capitula which terminate the tentacles in his Stauridium. ■ 

 Dujardin further compares them to the processes which are emitted by an Actinophrp or an 

 Acineta. These bodies, however, had been previously noticed by Corda,' where they lie over the 

 smaller kind of thread-cell which occurs in the tentacles of Hydra ; but he erroneously describes 

 them as direct prolongations from the thread-cell, so as to constitute with it a special organ, 

 which he calls an " organ of touch." They have also been examined by Dr. T. S. Wright,* 

 who follows Dujardin and Leydig in showing that they are not directly continuous with the 

 thread-cell, and who also maintains their very simple protoplasmic nature. He proposes for 

 them the name of " palpocils." I can entirely confirm the views of Dujardin, Leydig, and Wright 

 with regard to those bodies which I have examined in various hydroids (PI. IV, fig. 4). 



In the marine hydroids a very delicate structureless pellicle can usually be shown to 

 exist for a greater or less extent over those parts which are not covered by the ordinary 

 perisarc. I believe it to be either a simple excretion from the surface of the ectoderm or the 

 result of a metamorphosis of the most superficial portions of this structure. Sometimes, however, 

 the place of this pellicle is taken, at least on the tentacles, by an exceedingly thin layer of a 

 transparent semifluid substance, which seems to possess the properties of sarcode (woodcut, 

 fig. 48). The minute filaments just described as occurring over the site of the thread-cell would 

 seem to be mere continuations of this sarcode layer, which also frequently presents here and there 

 little conical elevations, which have no relation with the thread-cells, and whose summit is con- 

 tinued into a filament of extreme tenuity (woodcut, fig. 48 <?, e, <?). These filaments were first 

 pointed out by Wright, who includes them along with the organs of Corda under the common 

 name of " palpocils." 



Leydig is of opinion that there exists over the whole surface of Hydra, except the surface of 

 attachment of the foot-disc, a very thin homogeneous cuticle. I am inclined to believe that what 

 Leydig names cuticle is really the sarcode layer, which here, as in the marine hydroids, is con- 

 tinued into the palpocils. 



^ Miiller's ' Archiv,' 1854. 



' 'Ana. des Sci. Nat.,' 1843, p. 370. 



* Ibid., 2me serie, 1837, p. 363. 



*■ ' Proc. Roy. Phys. Soc. Edinb.,' vol. i, p. 311. 



