271] TURBELLARIA FROM THE MISSISSIPPI BASIN— HIGLEY 77 



in fact more conspicuous than in the adult, because of the transparency of the 

 parenchyma and also because it was relatively very much larger. The tracts 

 of rhabdites had also become heavy enough to be visible and most of the body 

 structure was perfect. When, by accident, the body waU of the parent was 

 broken, all the embryos pushed out and swam away, seemingly perfectly at 

 home. In the collections together with the adults a larger number of small 

 individuals always appeared. In size these outreached the unborn embryos 

 only a very little. Thus it seems that the young are retained in the body of 

 the mother until a very late stage in their development. 



With the exception of a few scattered individuals of Mesosioma ekrenbergii, 

 this is the largest rhabdocoel known in the United States. Examined super- 

 fically it might be a small planarian. However, the angled cylindrical body, 

 the depressed head, and the viviparous habit mark it as a typical and also 

 specialized Mesostoma. 



Mesostoma simplex nov. spec. 

 Figures 32 to 38, 40, and 42 



A number of specimens of this form appeared in a pond together with 

 Mesostoma angulare and others of the same family. In many respects they 

 closely resemble the more common species. That is, the general shape, the 

 pharynx rosette, the slender body, the black eyes and the anterior tracts of 

 rhabdites, differ very little from those of other species. In certain details 

 however, they vary to a surprising degree, being decidedly different from asso- 

 ciated types. 



Several points of general appearance are notable. The size, altho not at 

 all constant, is not over 1.5 nun. long by 0.45 broad and 0.3 in depth. This is 

 much below the average for other Mesostomas. In shape, the difference 

 between this and other species is more pronounced. The head end is rounded, 

 tapering only a very little, and not at all constricted or differentiated from the 

 body region. The diameter is about the same at all points except, of course, 

 there is some little narrowing toward the tail. The head end is contractile 

 and as the animal moves, it is turned from side to side and stretched in various 

 directions. This causes more or less change in general shape, the tendency 

 toward a pointing of the anterior tip showing most conspicuously. The ridge 

 dividing dorsal from ventral surface is well-developed around the head and 

 this often comes into view as the animal reaches up and out. The color is a 

 hght yellowish-brown and is usually lodged in the parench3ana. It is very even 

 all over the body, which is quite the opposite from the condition in Mesostoma 

 angtdare. The anterior end, even, is not the least different in tinge from the 

 rest. The integument is thick and clearly defined. It is transparent and of 

 nearly equal thickness all over except at the lateral ridge and in some special 

 anterior portions which seem to be more sensitive. The cilia are not heavy 

 and are evenly distributed over the whole surface. The rhabdites are scattered 

 in a layer throughout the body just beneath the epithelium. Then, also, 



