THE NEMERTINI 187 



is a " frontal organ " a single retractile papilla in Metanemertini 

 and in Eupolia ; or a group of three papillae in Cerebratulus and 

 Micrura. In all cases the papilla consists of ciliated cells, between 

 which there open the ducts of the "frontal gland" (cf. Acoela). 



Finally, in Carinella, there is present a " lateral organ " in the 

 form of a retractile papilla, close to the excretory pore on each 

 side, and recalling that of Capitellids. 



The vascular system (35) consists primarily of a pair of 

 lateral vessels (in Proto- and Meso-nemertini) extending along the 

 entire length of the worm, lying in the parenchyma, just above the 

 level of the lateral nerve. Anteriorly these vessels pass through 

 the nerve ring (Figs. II. p., XXVII. c) and unite at the tip of the 

 snout and in Carinella are also joined at the level of the brain 

 at the posterior end they are united by a supra-enteric commissure. 

 The anterior part of the lateral vessel in Carinella is dilated (b), and 

 a series of short vessels are given off to a longitudinal, lateral, 

 proboscis-sheath vessel. In the Metanemertini (Fig. XXVII. 2, m) 

 a median dorsal vessel arises from this intra-neural commissure 

 and passes backwards above the gut, to fall into the supra-anal 

 commissure. For a part of its course the dorsal vessel runs just 

 within the rhynchocoel, the epithelial lining of which is here 

 modified. Moreover, a series of transverse vessels unite these three 

 longitudinal ones, but no branches are given off by the system 

 no blindly ending vessels except in Malacobdella (Fig. XIII.). 



The Heteronemertini (and Hubrechtia) combine the three 

 longitudinal and transverse vessels of the Metanemertini with the 

 anterior dilated regions of the Protonemertini, from which several 

 pharyngeal vessels arise (Figs. III., XXVII. 3, 4). 



This system of vessels is entirely closed, and contains a 

 colourless fluid in which there float nucleated cells of fixed outline, 

 usually oval and flattened. In the majority these corpuscles 

 are colourless, but in a few (Amphiporus) they are red or yellow ; 

 in others yellowish with a green tinge ; and in Eiiborlasia they are 

 yellow, spotted with red, so that the blood appears red. The 

 yellow tint is due to haemoglobin. 



Details are wanting as to the way in which this "blood" or 

 " haemal fluid " (?) circulates in those forms in which only the 

 lateral vessels are present. According to Burger, the blood, in 

 Metanemertines, flows out of the dorsal, through the circular 

 vessels, into the lateral ones, returning to the dorsal vessel at each 

 end of the worm. But this is very unsatisfactory and uncertain. 

 As to the morphological nature of this vascular system, it seems 

 certain that it is not "coelom" as we understand the term 

 in the case of Annelids, etc.; it has none of the characters which 

 we associate with this cavity. It arises in the young worm 

 during the formation of the " imaginal discs," as a space or spaces 



