KH.MAKKAliLK .si'KCIE.S OF ECllIlJROIDS. -M 



The Circulatory System. — This is essentially of the same 

 arrangement as in otlier known J^ehinroids, except in a few 

 points. One remarkable feature of it consists in the unusual 

 length of the vessel (figs. 18, 1*.> and 23, d.v.}, a fact which 

 stands in relation with the posterior situation of the so-called 

 lieart. This describes as usual a curve embracing the alimentary 

 canal, — in the present species, at the posterior end of the midgut. 

 As before indicated, a large part of the dorsal vessel in front 

 of the crop is lixed to the body-wall by a mesentery-like sus- 

 pensory membrane (fig. 23, d.mX »Such a membrane is unknown 

 in other Echiuroids, the dorsal vessel running free in the body- 

 cavity throughout its entire course. The parietal insertion of the 

 said suspensory membrane is dorsal in the anteriormost part. 

 Posteriorly its course gradually changes into lateral on the right 

 side of the worm and finally into ventral. Soon after this, the 

 membrane becomes confluent with the right lateral mesentery 

 before described. More posteriorly from this point, the single 

 membrane formed by the confiiuence connects the dorsal vessel to 

 the dorsal surface of the œsophagus and no longer to the body- 

 wall. This condition obtains for a length of 10-20 mm. in about 

 the region marked with •::• in fig. 23. After that, the posterior 

 continuation of the membrane separates from Ijoth the dorsal 

 vessel and the œsophagus, and runs for a short distance with 

 free edges until it becomes continuous W'ith the ventral mesentery 

 (v.m.), which joins the crop to the ventral body-wall on the left 

 of the nerve-cord. From the point the suspensory membrane 

 leaves oft' the dorsal vessel, this runs of course free in the body- 

 cavity down to the heart, except at a single point 10-12 cm. 

 from the mouth. The point is marked with + in fig. 23. There 

 arises from the vessel a short and slender nmscle thread, which 



