618 IvECOKL) OF CUIiKENT RESEAKCUES KELATJNG TO 



diflferciitiated in various ways ; just as individual cells vary iu their 

 characters, so too may the germ-layers give rise iu various ways to 

 the tissues and organs. The work now to be done is to define for 

 each class of animals (1) how the primary layers of ectoblast and 

 endoblast are converted into the definite layers and organs ; and 

 (2) how the cells are histologically differentiated in the separate 

 layers. 



Structure of some Coralliaria.* — Among the Coralliaria the 

 Actinioi have been the best studied. The almost total deficiency 

 of facts concerning the microscopical structure of the other groujDS 

 decided M. C. Merejkowsky to undertake a special study of some 

 species common in the Bay of Naples. The following are his 

 results. 



The ectoderm is shown to consist of the following elements : — 

 1. Ordinary ectodermic cells of very elongated form, excessively 

 depressed and dilated at the upper extremity which is invariably 

 furnished with only a single cilium. 2. Cells like the last but trans- 

 formed at their base into an excessively long and slender filament, 

 sometimes provided with several inflations which may be called the 

 nervous Jilaments. 3. Epithelio-muscular elements composed of cells 

 like the first (but shorter and broader), united at their base to 

 musculai- fibrillas. 4. Nematocysts of two kinds, the larger ones often 

 surrounded by protoplasm with a nucleus and a long filament (nervous) 

 in the posterior part, the smaller ones of a different form and always 

 furnished with a long posterior filament ; the filament bears at j^laces 

 small knots. 5. Glandular cells always pyriform and with coarsely 

 granular contents. 



The mesoderm is an elastic and structureless membrane, varying 

 in thickness iu the different parts of the body. It forms longitudinal 

 protuberances upon the faces of two mesembryenthal septa which 

 unite at the surface of the stomach. The muscles which spread in a 

 single layer over it are longitudinal in the interior of the animal and 

 disposed in horizontal rings on the exterior. They are either long, 

 slightly flattened filaments, the relations of which to the other histo- 

 logical elements it is not easy to ascertain, or they are fibrillfe form- 

 ing a part of the ej)ithelio-muscular elements. 



Another very citrious element consists of cells of comparatively 

 large size, and excessively flattened, which ramify greatly and unite 

 with each other by their ramifications, and are filled with granular 

 contents, with nucleus and nucleolus. They are arranged in a layer 

 and rest immediately upon the outer surface of the elastic membrane. 

 From their form, habit, and position, the author has no doubt but 

 that they are nervous ganglia in which the numerous fibrillse of the 

 different ectodermic cells terminate. 



The entoderm is composed almost exclusively of very typical 

 epithelio-muscular cells. The epithelial cell is not so strongly 

 elongated as in the ectoderm, but Avith the base much dilated, and 

 with a single cilium at the extremity. The muscular fibril is very 



* ' Comi'tcs Eeudus/ xc. (1880) p. 1086. 



