632 R. J. HARVEY GIBSON ON THE 



epithelial cells in a fine-pointed end (PL CXLIX. fig. 14). The epithelium of the 

 surface is continuous with the epithelium lining the ocular pit. Just after 

 turning the edge of the pit the cuticle becomes thicker, and then becomes split 

 into two layers, which are farthest apart at the bottom of the pit, but which 

 are connected throughout by a series of columnar fibres which pass directly 

 from upper to the lower layers (PL CL. fig. 16). The cuticle is therefore 

 replaced by a latticework bounded on either side by a cuticle. The fibres are 

 straight and homogeneous. They vary in thickness from the finest threads 

 scarce visible to columns about half as thick as a columnar cell. From the 

 outer layer of the cuticle and projecting into the cavity of the pit, are a number 

 of very fine fibrils of variable length. They are extremely delicate, and are 

 often destroyed in the section-cutting. The inner layer of the cuticle, which is 

 much thinner than the outer and not so hyaline in appearance, is continuous 

 with the ends of long, narrow columnar cells, which are longer than those of 

 the surface of the ocular swelling. Their basal extremities taper to fine fibres, 

 and become lost in a dense feltwork (often so dense as to aprjear homogeneous) 

 which underlies the epithelium. The upper half of these cells is pigmented, 

 the dark granules of pigment being arranged round the cell, not generally in 

 the protoplasm (PL CL. fig. 16a). The nucleus occupies the lower half of the 

 cell, and is long and contains many granules. The general protoplasm of the 

 cell also is very granular. 



Continuous with the subepithelial layer is a dense network of fibres and of 

 cells, which are probably nervous. A secondary pit in the floor of the eye 

 pit is usually to be seen, but it contains no pigment, and the cuticle has here 

 regained its single nature and uniform thickness. Beneath the ocular swelling, 

 a secondary swelling is situated; a pit similar in form to that just described is 

 present, but neither the cuticle nor the epithelium show the differentiation 

 mentioned as occurring in the true ocular pit. 



The connective tissue of the ocular pit and of the secondary pit is loose, and 

 presents large vacuoles. (The relation of the eye to the nervous system is dis- 

 cussed under the Nervous system.) 



The most important researches on the nervous system are those of Brandt, 

 Spengel, Fraisse, and Lacaze Duthiers. 



Brandt {Bull. Acad. St Petersburg, 1869) gives a very full account of the 

 nervous system, describing in detail the system of buccal ganglia. His account 

 docs not differ in nuiny respects from that given above. He has, however, 

 fallen into error more especially with regard to the arrangement of the visceral 

 nerves. The recurrent nerve, also, he makes to spring from the musculo- 

 pallial, whereas it springs from the splanchnic. Spengel (Zeit.f. wissen. Zool, 

 xxxv. 382) has an important statement in reference to the arrangement of the 

 visceral nerves, and to their relation to the ctenidia, which has already been 



