450 Miscellaneous. 



On the Formation of the Calcareous Corpuscles in Holothurla. 

 By M. Edgard H^eotjakd. 



In studying the calcareous corpuscles in the DendroehirotsD, of 

 which a considerable number of species are found at Banyuls and at 

 Roscoff, I have been led to discover their mode of formation ; and, 

 though these elements present in each species a form so peculiar 

 that it serves as a criterion in their determination, I have found 

 that the original form of the corpuscle is everywhere the same in 

 these animals. 



A calcareous corpuscle has, in fact, as the basis of its formation, 

 a group of hexagonal prismatic cells, arranged in a single layer, in 

 a pavement-like form. 



Four adjacent cells, and in the first place their walls of contact, 

 serve as a centre of attraction for the calcareous molecules. Thus, 

 at first, we get a corpuscle in the form of an X. The calcareous 

 matter afterwards reaches the other lateral walls of these four cells ; 

 but the bases of the latter always remain free from any deposit. 

 The centre of each cell is occupied by the nucleus, and the presence 

 of this, preventing the calcareous deposit from spreading, explains 

 the existence of holes in their structures. The calcareous deposit 

 being more abundant along the edges of the hexagonal cells is the 

 cause of the existence of the projections which occur on the surface 

 of the corpuscles. 



I shall call these four cells the four fundamental cells of the cor- 

 puscle, and give the name of the fundament al calcareous corpuscle to 

 that which originates by the calcification of the lateral walls of 

 these four cells. As a type of this fundamental corpuscle maj' be 

 cited the normal corpuscle of Colochirus Lacazii. 



Because the mode of formation is subjected to the same law, it 

 does not follow that the fundamental calcareous corpuscle is identical 

 in all species. The size, relative or absolute, of the hexagonal cells, 

 the form and dimensions of the projections from the surface of the 

 corpuscle ars, in fact, matters which are fixed for each species, but 

 vary in diflerent species. 



When once a fundamental calcareous corpuscle is formed, a calcifi- 

 cation analogous to that above described gains the neighbouring- 

 hexagonal cells one after the other. According to the species this 

 calcification either equally affects all the cells which surround this 

 corpuscle, or only some of them. Thus we get corpuscles repre- 

 senting sometimes discs, sometimes laminae, and more or less elon- 

 gated and contorted. 



In general, among the holes in the more complex calcareous cor- 

 puscles, we may distinguish four, which, by their size and their 

 lozenge-shaped arrangement, strike one at once. They are precisely 

 those of the fundamental calcareous corpuscle. 



To sam up, in the Dendrochirotse : — (1) The calcareous corpuscles 

 have as their basis of formation a plane of hexagonal cells ; (2) only 

 four adjacent cells are concerned in the commencement of the for- 

 mation of every corpuscle, and thus furnish the fundamental calca- 

 reous corpuscle ; (3) the apertures in the corpuscles are caused by 

 the presence of the nuclei of the hexagonal cells; (4) the projections 

 of the surface of the corpuscles are formed along the edges of the 

 hexagonal cells. — Comptes Bendm, November 7, 1887, p. 875. 



