OLTOOTRRMA PRAMMITES. 243 



which on the one hand conimunicates with the blood-spaces 

 of the plerome, on the other hand ends blindly at the 

 knobbed distal extremity of the process. The single oi* 

 double laj^er of epithelial cells composing their walls is con- 

 tinuous with the external ectoderm, but differs from the latter 

 in not containing pigment granules. I was at first inclined 

 to regard these processes as sense organs, but as I have been 

 nnable to find any nerve-fibres in connection with them or 

 to distinguish any sense-cells on them, I must conclude that 

 they are simply vascular processes, serving partly to attach the 

 body-wall to the test, partly to aid in the nutrition of the latter. 



The characteristic pigmented ectodermal epithelium also 

 forms a lining to certain remarkable cavities which lie right 

 and left of the branchial sphincter muscle. The relations of 

 these cavities, which may possibly be artefacts due to con- 

 traction in spirit, will be described further on. 



The atrial epithelium differs considerably from the external 

 ectoderm, consisting of a pavement epithelium composed of 

 very mnch flattened polygonal cells, devoid of pigment 

 granules, and fitting closely to one another by their edges. 

 The transition from the pigmented external epithelium, in- 

 voluted along with the test into the atrial siphon, to the 

 unpigmented atrial epithelium is abrupt. In the neighbour- 

 hood of the anus the atrial epithelium is modified. There is 

 a tract of columnar, flagellated cells leading from the anus to 

 the atrial siphon. These cells are very similar in shape to 

 the curious flagellated cells forming the lining epithelium 

 of the branchial sac, which will be described in connection 

 with that organ. 



The Plerome. 



As described above, the body of Oligotrema must be 

 thought of as if there had been a solid mass of tissue (the 

 plerome) between the external ectoderm and the gut. Into 

 this mass the atrial cavity has insinuated itself and, spreading 

 right and left of the gut and branchial sac, has divided the 

 plerome into an outer layer, the ectoplerome, corresponding 



