23 1 ] TURBELLA RIA FROM THE MISSISSIPPI BASIN— HIGLE Y 37 



regular in general outline. The wall, of course, is folded more or less but the 

 folds are nearly the same size and follow one after the other about the same 

 distance apart. Within this wall, or rather very closely applied to the outer 

 surface, are numerous masses of cells. These are a httle longer than wide and 

 are scattered regularly over the whole intestine. Von Graff (1911) figures 

 intestinal glands for Stenostoma tenuicauda. These are much smaller, varying 

 somewhat but about one-half the size of the above and are more numerous, 

 showing about three times as many in each individual. They are quite flat 

 against the surface and protrude only slightly, while those of Stenostoma 

 tenuicauda are heavier and nearly globular. These glands are undoubtedly 

 digestive in function but their exact working has not been made out. 



Other details of specific value are the special sensory organs. The ciliated 

 pits on either side of the anterior lappet portion are small and shallow com- 

 pared with those of Stenostoma leucops or Stenostoma tenuicauda. They are 

 very near the end, half as far from the tip as the width at that point. They 

 are not so conspicuous as in most species, since the epithelium is not thick or 

 the cilia long and consequently the outlines are dim. The patelHform or light- 

 refracting organs, on the contrary, are very distinct and appear as rather large, 

 bright, almost iridescent spots directly back of the two sensory pits and on 

 a fine with the anterior edge of the mouth. They are not exactly round but 

 have a slightly angular outline, showing thru the clear parenchyma, almost like 

 two bright eyes. The size and brilliancy of these organs distinguishes them 

 from the small dull structures in other species. 



The zooid formation is another trait. The first fission plane is far posterior, 

 cutting off about one-third of the body length. Second and third divisions 

 are seldom found. The diameter externally is not changed until the internal 

 parts have been nearly completed. The parenchyma acts as a protective 

 cushion surrounding the partially developed brain and pharynx and at the 

 same time confines and pushes back the intestine until only a very attenuated 

 portion connects with the anterior zooid. 



The distinctive details may be summarized as: first, slender glandular 

 bodies occurring in the wall of the intestine; second, especially large and clear 

 patelliform bodies together with small sensory pits; third, an extremely con- 

 stricted connection betweeen phar}Tix and intestine; fourth, a cushion-like 

 mass of parenchyma protecting the anterior end of the second zooid; fifth, 

 the very regular cylindrical shape. 



Family MiCROSTOinDAE 

 This family name was proposed in 1907 by Alex. Luther to contain two sub- 

 families, the Microstominae and the Macrostominae. Microstoma was before 

 this time a genus of the Catenulidae, but the researches of Sekera (1888) and 

 Vejdovsky (1895) and especially of Luther himself made a new classification 

 necessary. The Macrostominae had been a group composed of three genera. 

 The interrelation of these two sub-famihes was made on the basis of the 



