Gatty Marine Laboratory 1 St. Andrews. 17 



in its lumen, the edge of which stains deeply. A reticulated 

 investment (neurilemma) separates it from the ventral 

 blood-vessel, and a firm layer of the same tissue roofs in the 

 entire area, the fibres of which closely link it on to the 

 alimentary canal immediately above. The neural canal soon 

 becomes as large as the section of the nerve, and, as 

 mentioned, it seems unnecessary to term it a :< giant fibre. " 



Cunningham * (1888) is inclined to regard the neural 

 canals as supporting structures, which prevent the nerve- 

 cords being bent at a sharp angle, and where they are highly 

 developed the cords are not separated from the epidermis. 

 He states they have a position similar to that of the noto- 

 chord in relation to the neurochord and aorta. He failed 

 to trace a connection between these canals and any ganglion- 

 cell, whilst admitting their homology with those of the 

 Errant annelids. 



In a section of a young Bispira stained with Ehrlich/s 

 hematoxylin, the cephalic ganglia are rather widely separated, 

 for they occupy the upper and outer border of the vestibule 

 leading to the mouth, and which has the outline dorsally 

 of the letter M. To the exterior is a pale belt free from 

 cells, then a band of muscular fibres inside the chordoid 

 layer with its investment, whilst the cuticle and hypoderm 

 form the superficial coverings. The chordoid cells are large, 

 distinct, and transparent, each with its nucleus, and some- 

 times with two, and they form at the level of the brain a 

 horseshoe guard on the dorso-lateral region, the ventral 

 aspect of the ganglia abutting to a large extent on the 

 mucous membrane of the vestibule, the isthmus between 

 them following the descending bars of the M in its progress 

 from side to side. Moreover, in contact with the isthmus 

 dorsally are the basement-membrane and the hypoderm of 

 the cephalic cul-de-sac in free communication with the sea 

 water. The organ thus is in a favourable position for 

 receiving impressions from the exterior as well as by its 

 nerve-trunks, whilst the elastic chordoid skeleton gives 

 sufficient protection. In the transverse sections the entire 

 ganglion on each side is dotted with deeply stained nerve- 

 cells, which perhaps are most numerous toward the surface, 

 and they extend into the nerve-trunks, leaving the organ, as 

 well as being distributed on the isthmus from side to side. 

 In some cases they are grouped in arcs with the pale neuroglia 

 between, as if pertaining to a lobule, but, as a rule, there is 

 little definition in this respect. Immediately behind, the 



* Quart. Journ. Micros. Sc. n. s. vol. xxviii. p. 275. 

 Ann. & Mag. N. Hist. Ser. 9. Vol. ii. 2 



