ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 61 



granular cells, staining deeply with osmic acid ; they are also found 

 in the endoderm and in the septa. At the base of the column 

 the gland-cells are absent, the pigment-cells less numerous ; fusiform 

 ciliated cells, probably nervous, and a few thread-cells, form almost 

 the whole of the ectoderm. Mesenteries mainly as in Malacodermata. 



Embryology of the Zoantharia, — Lacaze-Duthiers' conclusions as to 

 the development of the mesenteries are confirmed ; the so-called 

 mesoderm is derived from the ectoderm. 



In Actinia equina the axis is the only part of the mesenteries 

 derived from the ectoderm. The gastrula is formed by invagina- 

 tion, and consists of two layers of large cells ; the ectoderm-cells 

 become smaller and fusiform, and the alimentary canal is formed by a 

 kind of secondary invagination, the edges of the gastrula mouth 

 being turned inwards and downwards into the cavity, which becomes 

 filled with large vitelline masses. At a later stage the condition, 

 especially as regards the mesenteries, corresponds with that given by 

 Kowalevsky for Alcyonium palmatum ; a distinct band {inemhrana 

 propria) separates endoderm and ectoderm, from the former of which 

 the mesenteries are derived. The mesoderm is differentiated from 

 a granular zone which is formed at the base of the ectoderm. 



Cerianthus membranaceus. — In four-tentaculate larvte, the filamen- 

 tous bodies noticed by Kowalevsky were observed, and considered 

 to be mesenteric filaments. A membrana propria separates the endo- 

 and ectoderm. 



Balanophyllia regia. — The vermiform new-born larvae float or swim 

 rapidly; the costaa appear soon after birth. At the earliest free 

 stage there is no membrana propria ; it is found in six-tentaculate 

 larvfe; the endodermic cells are large, and contain great hyaline 

 vesicles, especially those in the centre of the body-cavity. The 

 CBSophagus is formed by the invagination of the ectoderm into the 

 endoderm. The mesoderm arises as in Actinia equina. 



Histological conclusions. — The author finds that the ectoderm of the 

 tentacles varies little in the Actinians,' owing to the constant presence 

 of thread-cells interspersed with cnidocils, but that of the body- walls 

 varies considerably ; a granular basal zone containing nerve-fibres is 

 a constant feature of it throughout. The endoderm is remarkably 

 simple, and scarcely differs from that of the larvee. The cnidocil- 

 elements differ from ciliated cells only by the absence of cilia ; they 

 occur in Sclerodermata and in most Malacodermata, most abundantly 

 in the tentacles. The ordinary ectodermic glandular cells differ from 

 those of the endoderm by the granular character of their contents ; they 

 empty by rupture of the cell-wall ; a hyaline form of gland-cell also 

 occurs in the ectoderm in some species. The pigment-cells may be 

 found in either of the three layers. The sensitive epithelial cells, 

 which occur widely in the group, consist essentially of a fine fibril ; 

 the thread-cells are intimately connected with them. There are three 

 types of the latter : (1) fusiform, with threads spirally rolled — the most 

 common, found in all the species ; (2) larger, with winding threads, 

 peculiar to Corynactis, Cerianthus, and the Sclerodermata ; (3) without 

 thread, but with a spirally barbed rod. Cerianthus possesses all these 



