TYPE PLATYHELMINTHES. 165 



arrangement. The various terminal branches of the nephridial 

 tubes are club-shaped and closed, a flame of cilia projecting 

 from the closed end into the lumen of the tube. The canals 

 and tubes are lined with ciliated cells, and are therefore inter- 

 cellular and not iutracellular, i.e., do not perforate cells, differ- 

 ing in this respect from the nephridia of other Platyhel- 

 minths. 



The blood-vascular system is peculiar to the Nemertea 

 among Platyhelrninths, and consists in the simple forms, such 

 as Carinella, of two lateral vessels which anteriorly open into 

 lacuuar spaces without definite walls. In the more highly 

 organized forms, however, three longitudinal trunks, two 

 lateral and one dorso-median, are present with definite and 

 sometimes muscular walls, and unite in a J_-shaped manner 

 at the posterior end of the body, while in front they may 

 either open into a system of lacuna?, or, as in Tetrastemma, 

 unite with each other as they do posteriorly, a perfectly closed 

 system thus resulting. Transverse connecting branches be- 

 tween the dorsal and lateral vessels occur in regular succes- 

 sion, a metamerism being again suggested. The blood-vessels 

 and lacijnee contain a fluid in which float round or elliptical 

 corpuscles, which in some of the higher forms have a red 

 color, due to the presence of haemoglobin. No heart or 

 special contractile organ is present, the blood being driven 

 through the vessels, without an}^ definite direction, by the 

 movements of the body. 



The occurrence of a blood- vascular system in the Nemerteans and its 

 character in the lowest members of the group suggests a mode of origin for 

 the system which agrees well with what may be deduced from embryological 

 observations on other forms. It may be supposed that in the primitive 

 Nemerteans a system of spaces filled with fluid existed, in which cells derived 

 from the parenchyma floated. These spaces would represent a simple 

 oelom, and were lacunar in character, lacking definite walls, the cir- 

 "ilation of the fluid they contained being very irregular. In time the 

 aces along the sides of the body might arrange themselves in a linear 

 . ianner, and might acquire definite walls, the rest of the spaces remaining 

 lacunar, when a condition resembling that in Carinella would ensue, the 

 arrangement found in higher forms resulting from the conversion of the 

 remaining lacunar spaces into vessels with definite walls. 



According to this view the blood-vascular system is to be regarded as 



