HEKATEROBRANGHUS RHRUBSOLII. 185 



all lie freely in the coelom. They are represented in the first 

 segment by the vessels going to the branchiae. In the posterior 

 region of the body, i. e. where there is the sinus, it is difficult to 

 say whether transverse vessels are present or not. In some 

 series of sections vessels may be seen here and there leaving 

 the upper part of the remaining sinus where the dorsal vessel 

 is just nipped off. More posteriorly., where the sinus is con- 

 tinuous all round the intestine, vessels may sometimes be seen 

 running from the ventral vessel (fig. 8, v. v.). These do not 

 appear to occur regularly in every segment, and they cannot be 

 seen at all in some series of sections which show the other 

 parts of the vascular system clearly. But it is difficult to say 

 whether they are really not present, or whether they are merely 

 contracted, and therefore not seen, or not recognised as blood- 

 vessels. We should not, therefore, be justified in concluding 

 that the sinus represents them, although this would seem not 

 unlikely in the most posterior region where the sinus is com- 

 plete. In other Polychsets where there is a sinus {e.g. Scali- 

 bregnia, Trophonria, Eumenia) transverse vessels running to it 

 from the ventral vessel are long and well marked.^ 



The blood flows from behind forwards in the sinus and 

 dorsal vessel, from in front backwards in the ventral vessel. 

 It is probably aerated both at the anus and in the branchiae on 

 the head-segment, and also to some extent in the "cephalic" 

 tentacles. It would be interesting to note whether all forms 

 that have a sinus round the intestine have also other indications 

 of an anal respiration. The blood is red, coloured probably 

 by haemoglobin. It contains, as far as I have seen, no cor- 

 puscles. 



Coelom and Nephridia. — The coelom is partially divided 

 into separate cavities by the septa, which are thin muscular 

 partitions between the segments coate3 with coelomic epithelium 

 on either side. They move backwards and forwards with the 

 intestine. There is a dorsal and ventral mesentery supporting 

 the intestine (fig. 7), and thus dividing the coelom longitudi- 



' See A. Wiren, ' BeitrJige ziir Auat. u. Hist. d. Auuelideu. Kduigl. Sv. 

 Vet. Akademiens Handlingar,' Bd. xxii, No. 1. 



VOL. XXXI, PART II. NEW SEE. N 



