684 PROCEEDINGS OF THE ACADEMY OK [Dec. , 



rior ends, which are dark greenish and contain large globules that 

 have an oily appearance, the secretion from the posterior gland 

 cells. The large blood vessels, in which the posterior ends of the 

 cerebral organs lie, are very noticeable in life, fig. 16, C.Org.V. 

 They are usually expanded, and the floating blood corpuscles may 

 be seen even with a low power. 



Each cerebral organ consists of the following parts: (1) the 

 ciliated pit, opening directly to the exterior; (2) the ciliated canal, 

 leading from the jDit to the anterior end of (3) the cerebral organ 

 proper. 



The ciliated pit is the most anterior part of the cerebral organ. 

 It is a flask-shaped cavity, lined with a ciliated epithelium that is 

 differentiated histologically into several regions which will be de- 

 scribed below. The pit opens directly on the surface of the head, 

 there being no lateral slits, and in life is usually widely expanded, 

 the long cilia beaiing vigorously. In fixed preparations the pit 

 has a small external aperture, a narrow neck and the inner flask- 

 shaped portion, Plate XLI, fig. 21, Cil.P. 



The ciliated canal, Cil.C, is a narrow duct, a continuation of 

 the inner end of the ciliated pit. It extends in the transverse 

 plane of the body from the ciliated pit to the anterior end of the 

 cerebral organ proper, then, making a sharp turn at right angles to 

 itself, it enters the cerebral organ and continues backward to the 

 posterior end, where it terminates blindly. 



The cerebral organ proper is a, pear-shaped structure, wide at 

 the anterior end and gradually becoming narrower at the posterior 

 extremity. The cerebral organ nerve, which arises ventrally from 

 the posterior end of the dorsal brain lobe, enters the anterior end 

 of the cerebral organ on the dorsal surface, just in front of the 

 entrance of the ciliated canal, and then ramifies throughout the 

 organ. 



The basement layer of the body epithelium is continued beneath 

 the epithelium of the ciliated pit and forms around the cerebral 

 organ an envelope of connective tissue, fig. 21, Cn.T.S., of a 

 thickness about equal to that of the inner neurilemma of the brain 

 lobes. In fig. 21 the thickness of the basement layer and cerebral 

 organ sheath is slightly exaggerated. The most anterior part of 

 the cerebral organ is completely surrounded by muscular tissue — 

 the inner longitudinal muscle on the inner surface, the circular 

 muscle on the outer side. 



