160 BULLETIN OF THE 



of which are narrower and stain much deeper, which possess a denser, 

 more highly refractive cell body. Figure 21, a, c, e, represents these 

 cells, which are seen in situ at cl. sns., Figure 23. These may be sensory 

 cells ; I was, however, unable to discover the sensory hairs described by 

 Selenka ('83, p. xvii) as found on the external surface ; these cells 

 certainly possessed merely such cilia as those adjacent. At the level 

 of the mouth there is a transition from these filamentous cells to the 

 columnar cells of the intestinal tract. This serves to fix the level of the 

 oral opening proper, which would otherwise be indefinite on account of 

 the various degrees of expansion or contraction of the animal. 



b. Migratory Corpuscles. 



Between these filamentous cells are found at varying heights highly re- 

 fractive spherical nuclei 4 fx in diameter. My attention was first called to 

 them in a preparation stained by Ilamann's carmine (Plate II. Fig. 15), 

 where they become prominent by reason of their being stained deeper than 

 the other nuclei. A more careful examination showed that they were 

 not accidental, as at first surmised, but definite independent structures. 

 Each is surrounded by an irregular clear zone varying in width from a 

 mere line to one half the diameter of the nucleus. By means of these 

 peculiarities, such cells were traced back through the cutis, where they 

 were most abundant in the spaces just below the basement membrane, 

 to the blood cavity, and were found to agree precisely in size and optical 

 character with one kind of blood corpuscle found in the coagulum there. 

 They may then be regarded as migratory corpuscles or leucocytes, 

 analogous perhaps to those of vertebrates. Similar cells are often met 

 ■with, though never in such numbers, throughout the body wall. 



The thin basement membrane to which the processes of the filamen- 

 tous cells are attached is not everywhere equally distinct. Owing to the 

 contraction of the different areas, it maybe thrown into extensive and 

 complicated folds, which, combined with the basal processes of the fila- 

 mentous cells, render its identification a matter of difficulty, but in 

 suitable regions it may be identified beyond a doubt. 



Beneath this membrane lies a cutis, very similar to that of the body 

 wall. It differs chiefly in the scarcity of pigment cells and in the en- 

 tire absence of glands. The " Wimperdriisen " seen by Vogt und Yung 

 ('88, p. 406) on the oral surface of the tentacles, are merely appearances 

 due to unequal contraction of certain areas, which produces structures 

 superficially similar to the sensory organs of the anterior papillate zone 

 already described. The cutis is further peculiar in the possession of 



