162 BULLETIN OF THE 



the various folds and plaits becomes at once clear. Furthermore, the 

 muscles concerned in emptying the blood cavity are primarily the power- 

 ful trabeculse and the longitudinal muscles, whereas the circular muscles, 

 which are comparatively scanty, are only of secondary importance. 



The cutis of the oral fold contains also numerous vacuoles in groups 

 near the basement membrane, and these may be seen in transverse sec- 

 tions filled with the migratoi-y cells previously mentioned. In addition 

 to these small leucocytes, occasional larger granular cells are found in 

 the lacunse. These correspond again to the granular corpuscles of the 

 blood. They do not make their way into the hypodermis. A tissue 

 which might be homologized with the supporting tissue of Phoronis does 

 not, according to my observations, exist in this form.^ 



Lastly, lining the blood cavity and covering the trabeculse is an endo- 

 thelium of very flat cells with proportionally large nuclei. This en- 

 dothelial lining is continuous, and is adjacent to a mass of gelatinous 

 connective tissue, which is without vacuoles, so that the blood corpuscles 

 could reach the hypodermis only by a definite migration through the 

 endothelium and the connective tissue. The cavity is often distended 

 by a coagulum which contains corpuscles that, as various writers have 

 maintained, actually differ in size from those of the coelomic fluid, so 

 that a connection between the two cavities was regarded as improbable. 

 I can confirm the statement of previous writers that no such connection 

 exists. Yet as the corpuscles are in size between the extremes of those 

 in the coelomic fluid, and not far from the average (cf. the exact meas- 

 urements of Brandt, '70, p. 3), it is not improbable, in view of the 

 migratory tendency of the corpuscles already described, that the coelomic 

 fluid receives its quantum from the blood system by the active emigra- 

 tion of the corpuscles which are foi'med in that system. This was con- 

 jectured by Brandt ('70, p. 24), who had found, however, no evidence of 

 such a tendency on the part of the corpuscles. The detailed and careful 

 account of the vascular system given by him has been overlooked by 

 many later investigators. 



d. Vascular System. 



In contradistinction to Sipunculus Gouldii and to Phascolosoma, the 

 blood cavities of S. nudus are not in the form of regular vessels, but 

 are indefinite lacunar spaces, traversed by trabeculse at in-egular inter- 

 vals and extending throughout the whole tentacular fold, everywhere 

 almost equally distant from the exterior. 



^ See Addendum. 



