4 THE VOYAGE OF H.M.S. CHALLENGER. 



which, according to the degree of their histological differentiation, may be homogeneous 

 and not enclosing cells, homogeneous and enclosing cells, or, finally, fibrous and containing 

 cells. The framework of connective tissue gives us an accurate figure of the corporeal 

 form of the Actinia even when the epithelial parts have been removed by maceration ; 

 from the standpoint of the " Blattertheorie," it must be termed the middle layer of the 

 body or mesoderm. 



All the lamellae of connective tissue are covered on either side by a single layer of 

 epithelial cells, which are distinguished by extraordinary length and thinness, and may, 

 moreover, be placed in different categories according to their different functions. The 

 most usual form is seen in the " supporting cells," in which, despite their fineness in an 

 isolated condition, we can recognise a distinct, triangular, basal expansion. The most 

 common after these are the " urticating cells " and " gland cells." In the former the body 

 is expanded by the presence of the thread, in the latter it is distended by glandular 

 secretion stored up in it. The form of the nematocysts, and the nature of the thread 

 contained in them is not the same everywhere, and may, perhaps, some day become of 

 systematic importance. The glandular secretion is also of different kinds ; it sometimes 

 fills the body of the cell, as a homogeneous, glassy mass, sometimes it is deposited as 

 a mass of closely compacted granules, greedily absorbing colouring matter. The fourth 

 form of cells is that of the " sense cells," which have the same fine, filamentous nature as 

 the supporting cells, from which, however, they can be distinguished in an isolated 

 condition by their central end giving off two or more fine nerve threads, which have a 

 tendency to become varicose. 



With the exception of the glandular cells, all the cells bear appendages at the 

 peripheral end ; the sense cells, and probably also the urticating cells, have fine, long, 

 tactile bristles, of which each cell usually possesses only one ; the supporting cells bear a 

 bunch of cilia, or a simple flagellum. Ciliated cells and flagellate cells may be present 

 in the same animal, e.g., in most Actiniae the ectodermal epithelium is made up of the 

 former, the endodermal of the latter, whilst in Cerianthus we find only flagellate cells. 

 We have as yet no satisfactory knowledge of the manner in which the two forms of 

 cells are distributed among the Actiniae. 



The epithelial coverings are derived immediately from the two primitive layers of cells 

 of the gastrula larva, the endoblast and the ectoblast, and in the developed animal are 

 therefore to be distinguished as separate body layers, as endoderm and ectoderm, even 

 when they hardly vary in their histological character. The ectoderm covers the outer 

 surface of the body and the inside of the oesophagus ; the endoderm covers everything 

 else, i.e., the inner wall of the wdiole coelenteron, and the inner spaces of the tentacles. 

 The supporting lamellae of the wall, of the oesophagus, &c., are therefore covered with 

 ectoderm on one side and with endoderm on the other ; the septa only form an 

 exception, as they bear endodermal epithelium on both sides. 



