58 ILLINOIS BIOLOGICAL MONOGRAPHS [252 



Connected with the reproductive system is a series of muscle cells having 

 to do with the control of the atrial pore and the discharge of sperm and egg 

 cells The atrial pore, situated at the tip of a papilla, is closed by the contrac- 

 tion of fibers lying near the ventral body surface. These are attached to the 

 skin at one end and to the pharynx or neighboring structures at the other. 

 In the walls of the reproductive organs themselves are flat thin layers of delicate 

 muscle made up of many cells lying parallel to one another and acting together 

 as a whole. 



Thus the muscular system has to do with the functioning of the several 

 organs or with the specialized reactions of the different parts, acting separately 

 and independently of each other. In this species, it reaches a greater degree of 

 contractility than is generally the case in other forms. 



PARENCHYMA 



Parenchyma, or mesenchyma as it is sometimes called, is the term used 

 to designate the tissue, or rather mass of cells, which fills the interstices between 

 the several organs. It forms, as it were, the packing for the important parts 

 which are thus buried and cushioned, fairly secure against at least minor in- 

 juries. 



The cells which compose the parenchyma are irregular and variable in 

 appearance. Soft and extremely pliable, spherical when free, very angular 

 when confijied, they depend entirely upon surrounding conditions for shape. 

 Since they are not limited to any part of the body space but accommodate 

 themselves equally well to the large open portions of head and neck region or 

 crowd into the infinitesimal cavities between intestinal cells, it is imperative 

 that they be capable of very readily assuming any form. Correlated with such 

 possibilities for variation is a very delicate and membranous cell wall. The 

 nucleus is large tho so clear as to be impossible to distinguish from the proto- 

 plasm around it except in cells which have escaped into the surrounding water. 

 The cytoplasm is thin, almost watery looking, and contains a few rather con- 

 spicuous granules, the whole structure being very simple. 



In functioning this type of cell serves several purposes. It takes the place 

 of a circulatory system acting as the conductor for transfer of food materials. 

 As it is in more or less direct contact with all the groups of cells in the whole 

 structure, it forms a basis of connection between the digestive tract and other 

 regions. Where more highly specialized cells would preclude the possibility 

 of such connection, these seem to be able to carry it on thru even a compara- 

 tively long series. 



Another point of importance is the support which is given to the several 

 body parts, the parenchyma acting in Heu of a skeletal system. Since there 

 is not even a cuticula to give stiffness to the integument, and since none of the 

 organs possess the necessary rigidity in themselves, the shape is maintained by 

 internal pressure. The animal is, so to speak, inflated so that it can hold its 



