SENSORY EPITHELIUM OF MICROSTOMA CAUDATUM. 45 



represents a modified region of the general body-epithelium. 

 This differentiation can be studied in specimens undergoing 

 binary fission. Fig. 2 represents a ciliated pit in a newly-forming 

 individual. In this it is shown that as the flat, pavement 

 epithelial cells of the general surface pass into the wall of the 

 embryonic pit they become taller until at the fundus or blind 

 end of the pit there are very tall, columnar cells. As differentia- 

 tion proceeds, however, we get a point of great systematic 

 interest that has not been made so far as we have been able 

 to determine. The cells at the fundus of the growing pit 

 continue to grow and differentiate themselves as unicellular 

 glands. Thus in the fully developed pit we have its fundus 

 composed of large, glandular cells which lie deeply embedded 

 in the mesoderm. These cells have pear-shaped bodies measuring 

 10 micra to 12 micra in diameter. Each cell has a glandular duct 

 leading into the lumen of the pit (Fig. 4, G'). It is not therefore 

 as von Graff ('09, Seit 64) says of rhabdoco-les that " Die Griib- 

 chenflecken sind Hautstellen, die keine Rabdoide und Driisenaus- 

 fiihrungsgange besitzen"; nor is the histology of Wilhelmi's 

 " Auricularsinnesorgane " described as being so differentiated. In 

 the mature ciliated pit of Microstoma caudatum Ldy. we have a 

 differentiation of its cells into a sensory and a glandular region 

 (Fig. 4). The apparent cuticula shown in this figure is but the 

 cut ends of cells radiating from the plane of the section, and it 

 cannot be considered analogous to the "homogeneous mass" 

 which Ott ('92) found covering the ciliated ends of the cells in 

 the ciliated pit of Stenostonia leucops O. Schm. 



As stated above this has considerable systematic importance. 

 Zoologists look upon the affinity between Turbellaria and 

 Nemertini as being very strong. The "cerebral organs" of 

 Nemertini have been considered the homologues of the ciliated 

 pits of Turbellaria. These "cerebral organs" in the highest 

 Nemertini show no resemblance to the ciliated pits of any 

 turbellarian but the "cerebral organ" "in its simplest form, in 

 the Protonemertini, is a mere groove in the epidermis not extend- 

 ing deeper than the basement membrane; it is lined by ciliated 

 cells, and at the bottom are large nerves from the brain" (Ben- 

 ham, '01, p. 185). So it appeared that the simplest "cerebral 



