514 SUMMABY OF CURRENT RESEARCHES RELATING TO 



of the sea bottom, of the surroundings, of the food, and of their 

 enemies, as determining the strength, size, and disposition of the 

 abactinal spines." 



Ccelenterata. 



Development of Calcareous Skeleton of Asteroides.* — Gr. v. Koch 

 having observed that between the crystalline calcareous substance 

 which forms the septa of Mussa and the hyaline connective substance 

 that surrounds it, there are cells which form a continuous layer, 

 asked himself whether these cells secrete the calcareous skeleton, and 

 do they belong to the connective substance (mesoderm), or are they 

 ectodermal in origin, and, therefore, a secretion from the primitively 

 external surface, which is only apparently internal ? 



To resolve these questions he has studied the development of 

 the skeleton in Asteroides, where he finds that the first indications 

 are only to be observed some time after the larva has become 

 fixed. These appear as a circular disk with a cavity in the centre, 

 consist chiefly of carbonate of lime, and are composed of spheroidal 

 pieces, made up of concentrically arranged layers. The spheroids 

 are larger in the centre, and decrease in size as they approach 

 the margin of the disk. This earliest skeletal rudiment lies between 

 the lower layer and the ectoderm of its aboral surface. The 

 cells of the latter, like those of all other parts of the surface of 

 the body, are cylindrical, and no calcareous concretions are to be 

 observed within them. From these facts the author concludes that 

 the first rudiment of the skeleton is neither a product of the endo- 

 derm nor of the connective substance (mesoderm), but that it is a 

 secreted product of the ectoderm. The further development of the 

 skeleton is brought about by the completion and enlargement of this 

 basal disk, and by the formation of septa. The former is effected by 

 the formation of new spheroids, by growth, and by subsequent fusion ; 

 the septa arise from radial endodermal ridges which attain a consider- 

 able size. Later on, the ectoderm enlarges, and there appear cal- 

 careous secretions, formed of small crystals, which fuse with the disk, 

 and form the first rudiments of the septa. In the next stage the septa 

 are higher and begin to branch at their peripheral ends. Growing 

 still more, they broaden out, partly into the form of thin lamellte, get 

 small spinous processes, and fusing at their peripheral ends with one 

 another form the theca. The columella is similarly formed by a fusion 

 of the central ends. While these changes have been going on there is 

 a further secretion of carbonate of lime at the free edge of the young 

 Asteroides. This unites with the aboral disk, and gives rise to a thin 

 lamella. This is the epitheca of authors, and it is important to note 

 that at first it is completely separate from the theca, and that it only 

 secondarily becomes connected with it. Very little now remains to be 

 observed, except the growth of the septa, theca, columella, and 

 epitheca. We may remark, however, that from the primitively twelve 

 septa there soon arise six which are stronger than the rest, and, alter- 

 nating with them, appear to give rise to two cycles ; twelve new septa 



* MT. Zool. Stat. Neapel, iii. (1882) pp. 284-92 (2 pis.). 



