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



Ectoderm. — In the Hexactinige the ectoderm is divided by 0. and R. Hertwig into 

 three layers : (1) a comparatively thick epithelial layer; (2) a nervous layer, which is 

 rendered granular by the action of most reagents ; and (3) a thin muscular layer, applied 

 to the outer surface of the mesoglcea. The epithelial layer contains four kinds of cells, 

 viz.: — Nematocysts, glandular cells, ciliated epithelial cells (Stiitzzellen), and sensory 

 cells. The two latter types call for a word of explanation. The ciliated epithelial cells 

 are narrow- and band-like, with a dilation at each extremity, but the peripheral end is 

 wider ; they bear numerous cilia, and generally extend from the surface of the ectoderm 

 to the muscular layer. The sensory cells are extremely fine and thread-like, with a 

 dilation containing the nucleus about the middle or near the base of the cell. They bear 

 at the apex a single delicate flagellum, and the base passes into numerous fine fibrillse, 

 which are connected wdth a nerve plexus. Ganglion cells with large nuclei are generally 

 recognisable in the nervous layer. The sensory cells are distributed with a considerable 

 degree of uniformity between the other cellular elements of the ectoderm, but owing to 

 their extreme delicacy little information concerning them can be gathered from sections, 

 aud it is necessary to separate the sensory cells and ciliated " Stiitzzellen " by maceration, 

 &c, before their outlines can be made out. 



The ectoderm of Antipaihella subpinnata, as might be expected, is not so complex as 

 that of Hexactinise, but it appears probable that it has a similar general structure. The 

 ectoderm of the tentacles is very thick, and its surface is raised into a large number of 

 transverse oval ridges, which vary from 85 x 45 /x to 48 x 22 jx in diameter. In longitudinal 

 sections of a tentacle these ridges appear as gentle crenations of the surface, which 

 become lost towards the apex of the tentacle. Most prominent amongst the histological 

 elements of the layer are a number of clusters of large hyaline cells, which extend from 

 the surface to the base of the ectoderm (PI. XIII. fig. 8, g'). These are probably gland 

 cells, and may serve to secrete mucus. They remain cpiite transparent in sections stained 

 with borax-carmine, and the outline of the individual cells is not well defined. Each 

 cell has a small round nucleus near its base. Near the middle of each group three or 

 four more deeply-stained lines may often be noticed at moderate intervals, which ajjpear 

 to commence at a little distance beneath the surface, and to reach nearly to the base. 

 The middle of each line bears an oval thickening, wdiick is also deeply stained. Bearing 

 in mind the inter-relationship of the various types of cells in the ectoderm of Actiniaria, 

 it appears probable that the deeply-stained lines with a median thickening represent 

 thread-like nucleated sensory cells, which are interposed between the gland cells. In 

 specimens stained with haematoxylin, the hyaline cells stain darker than the bundles of 

 nematocysts, but no further structure is shown, the stain being apparently confined to 

 the cell-wall and the nucleus. Each group appears to consist of rather slender elongate 

 cells, having a dilation at the surface of the layer. Sometimes the distal portion of a 

 cell is triangular in section. These hyaline gland cells are from 10 to GO jx long, and vary 



