122 



MORPHOLOGY. 



Fig. 53. 



In Gemmaria are also found certain remarkable pedunculated sacs which are developed from 

 the marginal tentacles of the medusa, and filled with thread-cells (PI. VII, figs. 3 and 4). Thread- 

 cells are also, in many cases, specially developed in the nematopliores- — those peculiar sarcode 

 appendages already described as characterising the family of the Plmnularido!. 



It is almost certain that the thread-cells are always developed in the interior of proper 

 cells, and they may be frequently separated from the ectoderm with the generating cell still 

 surrounding them. 



From observations which I have made on the larger kind of thread-cell in Hydra, it would 

 seem that a portion of the protoplasm of certain ectodermal cells (generating cells) becomes 



differentiated as a spherical or oval mass which may be seen 

 to. occupy a vacuole in the midst of the remaining proto- 

 plasmic contents of the cell (woodcut, fig. 53), and in which 

 one or more nuclei are usually apparent. This little mass is to 

 become developed into the thread-cell. It continues to in- 

 crease in size, and becomes invested by a distinct cell-wall, 

 while its contained protoplasm becomes, in a way which 1 

 have not been able to follow, metamorphosed into the proper 

 contents of the thread-cell. After this the now mature 

 thread-cell becomes free by the rupture of the cell in which it 

 had been generated.^ The development of the thread-cell 

 may thus to a certain extent admit of comparison with that 

 of spermatozoa — a comparison already made by Leuckart — 

 but we are still far from having a satisfactory notion of the 

 origin and mode of development of these remarkable bodies. 

 To the ectodermal structures also belong the sense- 

 These will be considered in the section on the physiology of the 



Development of thread-cell in Sydra. 



a. Generating cell of the thread-cell detached 

 from the ectoderm, containing a very pale 

 yellowish protoplasm, from which a small mass 

 has become separated, and occupies the interior 

 of a vacuole, where it looks like a nucleus in a 

 cell. 



b. The separated mass of protoplasm has 

 hecome larger and more oval ; two nucleolus- 

 like corpuscles were in this instance apparent 

 within it. 



c. A developed thread-cell occupies the place 

 of the protoplasm-mass in the vacuole of 

 a and h. 



organs — ocellus and lythocyst. 

 Hydroida. 



2. The Endoderm. 



Cellular structure of endoderm — contents of the cells. — The demonstration of a definite struc- 

 ture in the endoderm is much easier than in the ectoderm. Here, indeed, we seldom meet with 

 any difficulty in making out a very distinct cellular composition. 



Most usually, two sets of cells may be detected in the endoderm : an external set, with mostly 

 clear, colourless contents, having but few granules ; and an internal set, which forms the immediate 

 boundary of the somatic cavity, and contains abundance of coloured granules. 



The separation between these two sets of cells is often quite abrupt. In many hydroids 

 those composing the external set are large, elongated, and with their longer axis in the direction 

 of the radius of a transverse section of the hydioid, while those composing the internal set are 



' Miibius has noticed that iu the development of the thread-cells of Lucernaria and of certain 

 Aclinozoa amoeboid changes of form are occasionally exhibited by the generating cell. Op. cit., p. 10. 



