16 ARTHUR E. SHIPLEY. 



The Nervous System and Sense-Organs. 



The brain is a bilobed organ, continuous by its anterior 

 face with the ectoderm of the invaginated prseoral lobe, and 

 surrounded elsewhere by a process of the lophophoral blood- 

 vessel, from which it is separated, not only by the endothelium 

 of the vessel, but also by a connective-tissue capsule (see figs. 

 2, 4, 8, and 27). The groove between the two lobes is deepest 

 and widest on the anterior surface, where the substance of the 

 brain is continuous with that of the prseoral ectoderm. 



In the brain, as in the ventral nerve-cord, the ganglion^cells 

 are aggregated in the side nearest the skin ; they are on the 

 dorsal side of the animal in the brain, on the ventral in the 

 nervous system. 



As the figs. 24, 25, and 26 show, there is a cap of ganglion- 

 cells covering the anterior, dorsal, and posterior surfaces of the 

 brain. The ventral surface is not invaded by the ganglion- 

 cells ; but here the fibrous tissue, which makes up the rest of 

 the brain, comes in contact with the thin connective capsule. 

 It is this region of the brain which projects into the blood- 

 sinus. 



The majority of the ganglion-cells are small, with deeply 

 stained nuclei, occupying about one half of the cell; they are 

 either unipolar or bipolar. At the postero-dorsal angle of the 

 brain, however, a certain number of giant ganglion- cells are 

 found (fig. 27). These cells have a diameter of '02 mm., at 

 least four times that of the smaller cells ; their nuclei are rela- 

 tively smaller, and they are unipolar. I was unable to trace 

 what becomes of the fibres given off from these giant-cells. 

 No such giant-cells occur in any other part of the nervous 

 system. 



A pair of sense-organs, usually described as eyes, lie em- 

 bedded in the substance of the brain. 



Each of these sense-organs has the form of a long tube bent 

 upon itself, so that one limb is nearly at right angles to the 

 other. The outer limb, the lumen of which is narrow, opens on to 

 the surface of the prseoral lobe (figs. I and 25), the opening lies 



