38 MORPHOLOGY OF 



Robinii, and it will be better first to describe its features in 

 this form. Its structure is best seen by killing the tissue in a 

 mixture of one part of 1 per cent, osmic acid and one of sea- 

 water, then washing with sea-water, and staining with picrocar- 

 mine. This tissue on being teased out in glycerine shows the 

 structure figured in figs. 76 and TH . The cells are very long, 

 and most, if not all of them, extend the whole length of the 

 skin (cf. fig. 75). The heads of these cells in the natural 

 living state are closely in contact with each other, but on 

 pressing out the tissue both in living and also in preserved 

 specimens these heads may be stretched away frona each other, 

 but each remains attached to its neighbours' by more or less 

 regular anastomoses. It thus is brought about that the 

 surface of the skin is made up of a sort of honeycomb of 

 tissue, each of the nodes being the outer end of an ectoderm 

 cell. The cells are very difficult to separate finely, but the 

 skin may easily be broken up into small rectangular pieces. 

 On separation each cell is very thin ; its outer end is slightly 

 pyramidal, and is continued into a thin fibre which gives off 

 anastomoses with adjacent cells and dilates at intervals. In 

 one of these dilatations, generally the last, the nucleus is 

 placed. Below this point the cell is continued into a very fine 

 filament which may be traced for some distance. Many of 

 these filaments terminate in small round knobs, which are 

 possibly due to reagents. 



In sections of hardened specimens these filaments may be 

 followed into the layer of nerve- fibre, which is always more or 

 less developed at the base of the ectoderm cells over the whole 

 body. These cells compose the larger part of the skin of the 

 proboscis and collar. Amongst them are distributed cells 

 which probably secrete mucus, &c. These cells are of several 

 kinds. First, in the skin of the proboscis are large goblet cells 

 whose nucleus alone stains (fig. 75, mu'). Next, in the skin of 

 the back of the collar and of nearly all the rest of the body 

 excepting those parts in which concentrations of nervous 

 tissue are found, almost the whole tissue is made up of large 

 cells full of some substance probably lubricating also, which 



