576 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Vascular System of Oligochseta.* — S. Sterling has studied this in 

 Stenutus niveus, Euenchytrseus iisetosus, Eisenia (Allolohophora) fcetida, 

 Pheretinia rodericensis, and other forms. 



The ventral vessel arises as a space between the endoderm and the 

 still unsplit mesoderm. As the latter becomes more differentiated the 

 space is gradually surrounded by splanchnopleure cells. These meso- 

 dermic cells form the future walls of the vessels. 



The two lateral vessels arise in the same way as lateral clefts. 

 They are more dorsalwards in the anterior region and form the dorsal 

 vessel. 



The intestinal vascular plexus (or sinus) arises subsequently from 

 numerous longitudinal and circular clefts, whose walls are due to the 

 splanchnopleure. 



The septal vessels lie first in the intersegmental cavities, which com- 

 municate with the space of the ventral vessel. The nephridial vessels 

 seem to arise in close connection with the interseptal cavities. The 

 typhlosole vessel is in close connection with the dorsal vessel and the 

 sinus. 



The subneural vessel arises as a gap between the two halves of the 

 nerve-cord on the ventral side, and its walls arise from immigrant 

 mesoderm cells. 



The chloragogen is formed by differentiation of splanchnopleure 

 cells. The cells in the blood are hasmocytes, of mesenchymatous origin, 

 and multiply mitotically. The valves and vasochords are " exotropic " 

 structures, which arise very early in the lumina of the vessels, and have 

 an important mechanical function. 



In lower Oligochteta the intestinal sinus is a space between the in- 

 testinal epithelium and the musculature. It is lined by a homogeneous 

 membrane, lying directly on the circular muscle-layer. The dorsal 

 vessel has in its anterior an intima, corresponding to the basal membrane 

 of the sinus ; outside this there is longitudinal muscle, circular muscle, 

 and chloragogen. The internal cells in the hearts, often called mus- 

 cular, are amoeboid. The dorsal vessel has in its anterior region a vaso- 

 chord, probably mechanical in its function. In the ventral vessel and 

 the lateral vessels there is much the same structure as in the dorsal. 



In the higher Oligochasta, if there is an intestinal blood-sinus, it has 

 a basal membrane, but no vasothelium ; where there is a vascular net- 

 work instead of a sinus, the vessels have the same walls as in other parts 

 of the system. The (contractile) dorsal vessel is lined by an intima, 

 outside which there is longitudinal muscle, circular muscle, and peri- 

 toneum. The lateral hearts have the same structure. The (slightly 

 contractile) ventral vessel has an intima and a strong circular muscle- 

 layer. In some species there is a weak longitudinal muscle-layer. In 

 the dorsal and ventral vessels the musculature is imbedded in connective- 

 tissue, often strongly developed. There is no vasothelium, but blood- 

 cells are often lying against the walls in the larger vessels. 



Amcebocytes of Lumbricus.f — H. Joseph has made a detailed study 

 of these elements, but we cannot do more than refer to the main con- 



* Jen. Zeitschr. f. Naturw., xliv. (1909) pp. 253-352 (9 pis. and 16 figs.), 

 t Arbeit. Zool. Inst. Univ. Wien, xviii. (1909) p. 1-60 (3 pis. and 30 figs.). 



