OLIGOTREMA PSAMMITES. 241 



to the body-wall. In both cases it could be seen to con- 

 sist of a single layer of somewhat flattened, fusiform cells, 

 whose pointed extremities interdigitate. These cells are 

 easily recognisable because of the numerous minute granules 

 of black pigment scattered through their cytoplasm (6g. 29, 

 ect). Over the greater part of the surface of the body the 

 ectodermal epithelium appears to be simply a flat layer 

 applied to the internal surface of the test, but in the neigh- 

 bourhood of the bases of the arms and the branchial siphon 

 the relations between test and epithelium are more intimate. 

 In these regions the body-wall is produced into irregular 

 branching processes, covered with pigmented ectoderm, 

 which penetrate into the softer and apparently semi-fluid 

 internal layer of the test, as is shown in fig. 29. (In the 

 figure the processes in question are separated from the test 

 by shrinkage due to preservation in spirit.) In many of the 

 pigmented ectoderm-cells there are yellow granules which 

 are apparently formed in the ectoderm-cells themselves, for 

 I could not find any traces of similar granules in the amoobo- 

 cytes of the plerome in this region. But the amoebocytes 

 which are passing through the ectoderm or have passed 

 through it into the test are loaded with yellow granules 

 (fig. 29, amc). It seems that the amoebocytes — one of them 

 is shown clearly at amc — pick up the yellow granules formed 

 by the ectoderm, and when loaded with them migrate into 

 the test, pass through it, and discharge their contents on its 

 free surface, thus forming the sticky, flocculent deposit which 

 attaches grains of sand, sponge spicules, and other foreign 

 substances to the test. Similarly amoebocytes laden with 

 granules pass down the lumina of the hollow villous pro- 

 cesses of the test. 



The pigmented external ectoderm also enters into the 

 composition of a peculiar form of tissue found on the ventral 

 sides of the bases of the arms. The diagram, fig. 34, shows 

 that near the bases of the arms there are recesses or indenta- 

 tions of the body-wall, and that the space between test and 

 body-wall is prolonged some way into the body. These 



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