8o SCIENCE PROGRESS. 



parenchyma so that we do not find a sheet of contiguous 

 epithelial cells on the inner side of the cuticle, but, instead, 

 a thin layer of the parenchyma which in common with other 

 authors he calls the basement membrane (Fig. I. d). 

 Situated below this, though at slightly different levels, are 

 a number of spindle-shaped cells with well-marked nuclei, 

 these represent the external epithelial layer of other animals 

 (Fig. I. c). Although the bodies of these cells are separated 

 from the cuticle by the basement-membrane mentioned above, 

 they have not lost their original connection with the ex- 

 ternal surface, but send up fine processes, often several from 

 one cell, which pass through the basement membrane and 

 terminate in the cuticle (Fig. I. £■). The lateral separation 

 of epithelial cells from one another which is here seen to be 

 due to the intrusion of the parenchyma is paralleled in the 

 case of the leech, where the connective tissue, often carry- 

 ing with it minute blood-vessels, penetrates between the cells 

 of the ectoderm. This feature is particularly well marked 

 in preparations made from the body wall of a leech which 

 has recently enjoyed a full meal. 



The parenchmya of Cestodes, like other connective 

 tissues, consists of cells and a ground substance. The cells 

 are very richly branched (Fig. I. d), their finer ramifications 

 stretching through the body in all directions, surrounding 

 all the organs and often accompanying for some distance 

 the muscle fibres. Their processes anastomose with those 

 of other cells such as the epithelial cells and thus the paren- 

 chyma cells are in a position to play a large part in the nutri- 

 tion of the parasite. They secrete the ground substance which 

 they throw off in the form of fine membranes and strandsform- 

 ing a network in the meshes of which is a slightly coagulable, 

 non-corpusculated fluid. This network penetrates every- 

 where between the epithelial and other cells, it surrounds 

 the muscle fibres and even accompanies the branches of the 

 nerve-cells as far as the cuticle where however the nerve 

 fibres end freely. Certain of the cells of the parenchyma 

 have become specialised to secrete the calcareous bodies 

 (Fig. I e), and in these cases the secretion has much the 

 same relation to the cell as the fat drop has to its cell in 



