1901.] NATURAL SCIENCES OF PIIILADELrillA. 685 



The change in the relative position of the circular muscle in 

 respect to the dorsal brain lobes and the cerebral organs has 

 already been described, the circular muscle lying on the inner side 

 of the dorsal lobe, but, with the beginning of the cerebral organs, 

 bending out so as to adjoin their outer surfaces (cf. figs, 19 and 

 21). 



From fig. 16 it may be seen that the cerebral organ vessels, 

 C.org. v., lateral branches from the median vessel, J/. V., run for- 

 ward, partly encircling the cerebral organs and ending blindly near 

 their anterior ends. The anterior part of the cerebral organ (see 

 fig. 21, right side) is but partly surrounded by the blood vessel, 

 while the posterior end,* fig. 21, left side, lies nearly free in the 

 blood vessel, being attached at the extreme tip to the body wall. A 

 noticeable thinning of the connective tissue envelope accompanies 

 the increase of the surface in contact with the blood vessel, and at 

 its posterior end the cerebral organ is covered only by a low 

 epithelium of square flattened cells, except at the point of attach- 

 ment to the muscular wall, where a small portion of the connective 

 tissue sheath persists, fig. 21, left side. 



Histology. — The epithelium of the ciliated pit consists of three 

 sharply differentiated regions: (1) the epithelium of the outer 

 part, or the neck of the pit; (2) the epithelium lining the median 

 jDart of the pit; (3) the innermost epithelium, 'adjoining that of 

 the ciliated canal. These three regions may be seen in fig. 21, 

 Oil. P. The epithelial cells have the same general structure in all 

 three regions, except that the cilia of the cells in the median part 



(2) are much longer. A cell from the median part (2) is shown 

 in fig. 7. It is a slender cell, with an expanded distal end on 

 which the long cilia, CiL, are borne, and tapering into a fine stalk 

 at the proximal bae^^al end; the small nucleus, N., lies just above 

 the stalk. Each cilium consists of a basal knob, an upper knob 

 and a ciliary thread. 



Between the ciliated supporting cells of the outer (1) and inner 



(3) regions, numerous large interstitial connective tissue cells are 

 present, and their nuclei, fig. 21, Cn.T.N., are very noticeable in 

 sections. The median (2) region is characterized by its longer 



* The section drawn in fig. 21 is rather obliquely cut, so that on the 

 right the plane of the section passes through the ciliated pit and the begin- 

 ning of the cerebral organ, while on the left only the posterior part of the 

 cerebral organ is seen. 



