34 ILUNOIS BIOLOGICAL MONOGRAPHS [228 



processes extending in all directions and forming a connection between neigh- 

 boring parts. The simpHcity of this system is one of the characters of the genus 

 and this species seems to have fewer parenchymal cells than any other. 



The digestive tract is the most noticeable part of the anatomy. The mouth 

 with its enormous stomodeal indentation is conspicuous. The comparative 

 size of this hollow epithelial-lined portion is suggestive. The mouth proper is 

 situated at the inner end of it and is the point where the true ectoderm ends. 

 It is, of course, flexible and is controlled by a few strands of muscles. In the 

 main, however, it opens or closes as a result of the expansion or contraction 

 of the pharynx. The pharynx, when expanded to its limit, reaches the body 

 wall and even distends it, making the whole of that region appear round. This 

 amount of enlargement is greater than in any of the other common species. 

 In other respects it resembles them, as in the presence of gland cells connected 

 with the outer wall and the lining cilia. The entrance to the intestine is not 

 as sharply marked off as in some species and this opening is not precisely 

 governed as is the mouth. A very few muscle cells surround it but they are 

 not strong enough to act as a sphincter so that the closure is made by the 

 pharynx. The intestinal wall is perhaps the most specialized part of the body 

 structure; altho made up of only one thick layer of cells it shows a surprising 

 amount of variation. When not inflated by a large amount of food, it is thrown 

 up into a series of rather regular folds. The outer portions of the cells imder 

 certain physiological conditions show a protoplasm very finely granular, in 

 fact almost clear. It is these portions which undergo most of the pressure 

 and stress when the intestinal shape varies, and they generally are much 

 narrower and smaller than the inner half of the wall. This inner border is 

 often very irregular, some ceUs being pushed far out into the intestinal cavity. 

 This is caused either by the crowding of the outer margin of the wall or by the 

 internal pressure of the cytoplasm. For, as the assimilation process progresses, 

 the protoplasmic portions of each cell acquire relatively large amounts of food 

 material, generally in the shape of oil globules. Sometimes the contained mass 

 is so large as to occupy the major portion of the cell and gives it a very charac- 

 teristic appearance. Other cells so situated that only a narrow section of the 

 inner surface is free may have no extra material and be small and shrunken. 

 Thus in most respects the intestine is an organ very like that of other species. 



By shghtly flattening the animal the simple excretory tubule can be seen 

 contracting slowly and irregularly. It is large enough to be clearly distin- 

 guishable and its course can be followed from the posterior part forward to the 

 anterior loop and then back to the external opening. The diameter is about 

 that of the thickness of the integument and the white color makes it stand out 

 against the dorsal wall of the intestine. 



The reproductive organs have not been studied since the period of maturity 

 occurs rather seldom. Yet asexual budding is common. All of the speci- 

 mens taken were either in the process of forming zooid chains or had evidently 

 just separated. The first or median fission plane was the only one developed 



