-STRUCTURE ANfD DEVELOPMENT OE COELOPLANA. 13 



surface the epidermis is generally 20-25 l^ thick, while in the peripheral 

 parts it is somewhat thicker, measuring 25-30//. As is pointed out by 

 Abbott ('07), it is thinnest (ro-i8//) in the parts directly over the 

 tentacle-sheath. 



The ventral epidermis (figs. 2, 3) differs somewhat from the dorsal 

 epidermis in its histological character. It consists of the gland cells and 

 of the ciliated cells. Of the gland cells, one can distinguish the same 

 two kinds that we have seen in the dorsal epidermis. The granular 

 cells {g. g) are more rarely found in the ventral epidermis than in the 

 dorsal. Moreover, there occurs a fairly larger variation in the size of 

 the spherules contained in them : some granular cells may show spherules 

 quite as large as those found in the granular cells of the dorsal epidermis, 

 while others (fig. 2, g.g) may contain much finer spherules measuring 

 only i-i.$fJ. in diameter. The clear cells {c. g) are rather commonly 

 found in the ventral epidermis and are usually of size and of staining 

 reaction standing between the two kinds of the clear cells in the dorsal 

 epidermis. They stain fairly well with basic dyes and enclose in the 

 cell-body a strongly stainable network of fibrils. Some of the clear cells 

 were observed to have been fixed just in the process of the discharge 

 of the inclusion. The matter discharged, which is probably mucous 

 substance, is frequently seen covering the ventral surface of the body. 

 Samassa ('92) is of opinion that, all the kinds of gland cells in the 

 epidermis of ctenophores are no more than different phases of one and 

 the same kind of cell. This view was accepted by many subsequent 

 writers including Abbott ('07). Evidently, however, some, at least, of 

 the clear cells of Cocloplana secrete their inclusion actually, without 

 changing into the granular cells. Direct evidence for this is given just 

 above. Moreover, this seems to be supported by the fact that, on the 

 ventral surface of the body which is extremely slimy, the clear cell is 

 found much more commonly than the granular cell. 



The ciliated cell {c. c) is usually columnar in form, showing the 

 nucleus at varying point of its height, in parts where the cell-body is 

 broadest. The appearance of the nucleus resembles closely that of the 

 interstitial cell in the dorsal epidermis. In the surface view of fresh 

 material (fig. 5), the ventral epidermis shows ciliation everywhere, which 

 obscures the existence of gland cells. 



At the margin of the body skirt, the ventral epidermis is thickest, 

 tlic thickness exceeding somewhat that of the dorsal epidermis at the 

 same place. In all other parts, the ventral epidermis is generally, if 



