LINEUS. 245 



possible that careful histological work may demonstrate 

 the presence of ciliated cells modified to form sense cells 

 somewhat resembling the rods of the vertebrate eye. 



The basement membrane is structureless, and is com- 

 posed of the intercellular substance around connective 

 tissue cells. It takes a deep stain with hematoxylin or 

 nigrosin, though it is apparently unaffected by the carmine 

 dyes. Beneath the basement membrane is found a thin 

 layer of fine circular muscle fibrils, and underneath those 

 again a diffuse layer of small glands, each composed of 

 several cells. The secretion of these cutis glands stains 

 deeply with carmine or hematoxylin, and by it the minute 

 ducts of the glands may be traced to their external open- 

 ings. Mixed up with the cutis glands are numbers of 

 connective tissue cells of a peculiar form containing pig- 

 ment. These will be referred to later among the connec- 

 tive tissues. Around the cutis glands are also found the 

 most external fibres of the external longitudinal muscle 

 layer. The fibres are separated into a number of small 

 bundles by the connective tissue, a structureless invest- 

 ment exhibiting the same staining affinities as the base- 

 ment membrane, with which it is probably identical as 

 regards composition. 



Separating the outer longitudinal and the circular 

 muscle layers (PI. II., figs. 1 and 2, PL III., fig. 8) 

 is a tunic of nervous tissue, consisting of fibrils given 

 off from the lateral nerve cords, which also lie between 

 the same two muscle layers. The circular muscle layer is on 

 the whole not so thick as the outer longitudinal layer, and 

 of about the same thickness as the inner longitudinal 

 layer which it immediately invests. Towards the posterior 

 portion of the body the outer longitudinal muscle layer 

 becomes greatly diminished, whilst the internal longi- 

 tudinal layer here surpasses the circular la} r er in thickness. 



