MUSEUM OF COMPARATIVE ZOOLOGr. 9 



macerated material that one can learn what the true characters of this 

 tissue are. 



The hypodermis is thickest on the dorsal surface ; it becomes thinner 

 toward the edges of the body,* and in passing around to the ventral sur- 

 face it still continues to become thinner as far as the middle line, where, 

 forming part of the floor of the pharyngeal cavity, it reaches its greatest 

 attenuation. There are hypodermal thickenings around the oral and 

 genital openings, and also over two sensory areas on the ventral surface 

 of the head region, which will be described in another place. 



It is almost impossible to find a region where the cells of the hypo- 

 dermis are not modified by the presence of the dermal rods. In order 

 to get at the natural appearance of the cells, it is necessary therefore to 

 study them in young specimens, and in the region where the rods 

 are fewest; this region I have found to be near the margin, on the 

 dorsal side. Very thin cross sections of young individuals are the most 

 favorable ones for this purpose. 



The cells are columnar, the height necessarily varying with the thick- 

 ness of the hypodermis. The nuclei are large, have an irregular or 

 sinuous outline, and are situated, as a rule, near the bases of the cells 

 (Figs. 1 and 2). This position is not constant, and depends upon the 

 number and influence of the rhabditi that are present. There is no 

 nucleolus proper, the chromatin being scattered through the nucleus 

 in many large granules. The size of the nucleus does not appear to 

 depend upon the size of the cells ; for while the cells in different regions 

 vary to a great extent, the nuclei remain of nearly uniform size. 



The cells are finely striated ; the striations are most prominent at the 

 basal ends of the cells, and cannot be traced to their free ends. Such 

 radial striations have been described by Bohmig ('86, p. 294) in the 

 hypodermis of Grafl&Ua, and more recently by Lippitsch ('90, p. 328) in 

 the epidermal cells of Derostomum. lijima ('84, p. 369) also alludes 

 to fine striations m the epidermal cells of Planaria polychroa. The 

 cells do not "etwa flach auf die Basalmembran aufsitzen," but are con- 

 nected with it by fine processes " welche etwa kammfdrmig ziemlich dicht 

 neben einander stehen." These processes he believes to be directly con- 

 tinuous with the striations of the cells, and to be protoplasmic prolon- 

 gations of the cells. He traces them through the basement membrane 

 into the muscles below, thus establishing " eine organische Verbindung 

 zwischen dem Epithel und den Korperinnem." His figure (Taf. XX. 

 Fig. 4) is confusing, and in addition was drawn, as he admits, from a 

 specimen in which the basement membrane exhibited pathological con- 



