Professor Herdman on the Structure of Sarcodictyon. 41 



fusiform connective-tissue cells, and penetrated in all direc- 

 tions by a system of branching and anastomosing tubules lined 

 by endoderm cells, and continuous with the body cavity. 

 There are also a large number of spaces in the matrix of the 

 mesoderm, simple lacunse which lie between the endoderm 

 tubules (see PL II., Fig. 16, /ftc.),and are found chiefly in the 

 outer part of the body wall (PL III., Fig. 1, lac). Probably 

 these spaces communicate with the tubules. 



The mesoderm also contains curiously shaped calcareous 

 spicules, which are rare in the upper parts of the body 

 wall, but abundant in the lower parts and in the stolon, 

 where they form an almost continuous layer, and add greatly 

 to the strength and stiffness of the tissues. These spicules 

 are of a dull red colour, and give to the body wall, and thus 

 to the whole colony, its characteristic hue. In the pale 

 variety, from Loch Fyne, the spicules are colourless, and the 

 slight tint of the colony is due to the soft tissues of the 

 animal. 



The spicules vary greatly in shape, sometimes they 

 are rod-like, sometimes branched or forked, and sometimes 

 plate-like, but usually more or less echinated or spinose. I 

 have been able by examining an immense number, a selec- 

 tion from which are represented on PL II., to make out 

 that aU the different forms are produced by the union of a 

 few simple spicules shaped like a wedge, or more exactly 

 like a quadrilateral terminated by an isosceles triangle. 

 This simple form is shown at Fig. 3 on PL 11. 



In the young growing point of the stolon of a specimen 

 (see PL I., Fig. 4) of the yellow variety from Loch Fyne, I 

 found the comparatively simple spicules shown in Figs. 1 and 

 2 on PL II. Fig. 1 shows these spicules in their relative 

 positions, magnified about 50 times. Fig. 2 gives a few of 

 them more highly magnified to show their composition out of 

 simple wedges, a is formed of two rods. In h the rods are 

 spinose. In c they have bifurcated, d, e, and /, are formed 

 of irregular wedges. In g three wedges have joined, h also 

 shows three wedges with a space for the narrow end of a 

 fourth. In i four wedges have united symmetrically. In j 

 and h they are more irregular. While in I they form a cross, 



