42 bulletin: museum of comparative zoology. 



imbedded in pai^encliyma, occurs a layer of large columnar paren- 

 chyma cells with large nuclei. Several layers of similar cells often 

 occur around the dorsal vessel when inside the rhynchocoel. The 

 transverse vessels and many of the lacunae are without muscular 

 walls, and even where muscles occur the circulation of the blood is 

 dependent mainly on the movements of the body as a whole, and as 

 a rule passes backward and forward irregularly in any of the vessels. 

 In Stichostemma and Geonemertes large and deeply staining cells 

 occur at intervals just external to the lining of the vessels. Bohmig 

 ('98), suggests that these cells project into the lumen of the blood 

 vessel and thus direct the course of the flow of blood. Further evi- 

 dence, however, indicates that they are merely gland cells which 

 pour their secretion into the blood vessels. 



The Idood itself consists of a colorless plasma in which float 

 numerous corpuscles. These are usually oval in outline, but much 

 flattened and disc-like. Others are more nearly circular discs, and 

 some spheroidal or somewhat irregular in shape, but as a rule they 

 do not show amoeboid movements. 



In certain species ( Tetrastemma bicolor, T. signifer^ T. nigri- 

 frons, Arnj^hiporus cruentatus, and A. pulcher) the corpuscles are 

 oval and discoid and of a homogeneous red color, so that they very 

 closely resemble the red blood corpuscles of the fi'og. Each cor- 

 puscle is provided with an oval or spheroidal nucleus of moderate 

 size. Tliis is commonly situated somewhat eccentrically (PI. 14, 

 fig. 86). In certain forms graniiles of pigment or vacuoles of secre- 

 tion may also occur. In Euborlasia the corpuscles have fine red 

 dots scattered through a greenish stroma. In nearly all species, 

 however, the corpuscles are colorless, although they may have a 

 greenish or orange tinge in certain forms. 



Kephridial System. 



The excretory organs of the nemerteans consist usually of a single 

 pair of long, branched tubules, situated in close proximity to the lateral 

 blood vessels, and having one or more openings to the exterior on 

 each lateral surface of the body (PI. 7, fig. 56 ; PL 8, fig. 57 ; PI. 13, 

 fig. 82 ; PI. 14, figs. 83-85 ; PL 15, figs. 91, 92 ; PL 19, fig. 120). 



In the majority of forms the nephridia are limited to the esopha- 

 geal region, and usually extend through but a small ])ortion of the 



