ERYLUS ROTUNDUS. 293 



which were more or less in the shade, and am inchned to ascribe the differences 

 in the degree of pigmentation of the darkest parts of different specimens to 

 differences in the amount of light due to differences in the depth at which they 

 grew. Unfortunately the information about the depths given is not sufficiently 

 exact to allow of a definite conclusion on this point. 



The superficial j)art of the body is differentiated to form a cortex, composed 

 of an outer and an inner layer. The outer layer is occupied by dense masses of 

 spicules and appears as an armour. Under the outer exposed parts of the sur- 

 face this armour usually is 65-90 ji, in the walls of sheltered cavities, extending 

 farther into the interior, only 35 ji thick, or even thinner. Pigment cells 

 occur in the armour- between the spicules on the dark parts of the surface. The 

 inner la3'er of the cortex is usually 55-75 [x thick and contains hardly any 

 spicules. It is composed of paratangential fibres, pigment cells, and usually 

 contains also granule cells. 



The pigment cells, the number of which is in proportion to the degree of 

 darkness of the surface, arc nearly always elongate and usually extend para- 

 tangentially. They have one or, more frequently, several lobose or filiform 

 processes, appear irregularly amoeboid, and are very variable in size, 6-29 ^t 

 long. The transparent plasm of these cells contains numerous apparently 

 spherical granules, dark brown in transmitted light, which measure 0.3-0.8 /t 

 in diameter. These granules are usually rather uniformly distributed through- 

 out the body of the cell and its processes, but sometimes parts of the cell are 

 free from them. Occasionally rows of single pigment granules, appearing like 

 strings of bead.s, have been observed in the sections. These probably lie in 

 (invisible) filiform processes of pigment cells. 



In the distal part of the choanosome and in the lower layer of the cortex 

 of forms A and B of var. typica, and also in some of the othei-s, remarkable 

 granule cells have been observed in large numbers. These cells ajjpear to be 

 situated in spherical, oval, or irregular cavities of the ground substance, 15 20 /< 

 in diameter, which in some places lie very close together. The granule cells 

 themselves are more or less spherical, measure 8-12 /i in diameter, appear hya- 

 line, and stain slightly with haematoxylin and aniline-blue. They are either 

 simple and structureless, or composed of a number, from ten to twenty or so, of 

 polyedric parts 2-4 /i in diameter. The spaces between these parts appear to 

 be empty. Rarely a more strongly stained, superficial layer and a body, which 

 may be a nucleus, have been observed in the simple, undivided cells^ and occa- 

 sionally niinute pigment granules are attached to, or contained in, the ones 



