IV VERMES— BLOOD-VASGULAB SYSTEM 253 



again divides into two branches, which encircle the intestine and again unite in the 

 dorsal middle line to form a dorsal vessel running to the extreme posterior end of 

 the body. In the Cirratulidce {Ohcetozoiie), at the posterior region of the body where 

 there are no gills, a separate dorsal vessel and an enteric vascular plexus are both 

 wanting. They are here replaced by a blood sinus which continuously surrounds 

 the intestine inside its muscular wall. This sinus is continued anteriorly into a 

 strong pulsating dorsal vessel (heart) which runs through the whole branchial region 

 and gives off vessels to the gills directly or indirectly. In the Serpulidce the enteric 

 canal is generally embedded along its whole length in a blood sinus, and a dorsal 

 vessel is wanting. Lateral vessels frequently run along the sides of the intestine, 

 and are connected by segmentally arranged transverse loops with the ventral vessel, 

 and in the case of Serpulidce provided with a thoracic membrane these lateral 

 vessels give off branches segmentally to this membrane which break up in it into 

 extraordinarily numerous and fine ramifications. In the Serpulidce, where the gills 

 are developed exclusively in the head segment, each branchial filament is penetrated 

 by a single vessel, and the afferent and efferent blood passages first separate at the 

 bases of the gills. In nearly all other PolycJueta we can distinguish afferent and 

 efferent vessels (branchial arteries and branchial veins) which pass into each other 

 at the ends of the branchial filaments. In the CajiiteUidce and Glymridce a blood- 

 vascular system separate fi-om the body cavity is wanting. The blood mingles with 

 the ccelomic fluid (hjemolymph). In Mastohranchus alone there are still found 

 rudiments of a' blood sinus surrounding the intestine. In many PolycJueta there is 

 found, in that broader part of the dorsal vessel which is called the heart, a strand, 

 generally of a brown colour, lying freely in its lumen. This is called the heart body. 

 Its function is not yet clear. In Chcutozone 3 such bodies are found. 



The blood-vascular system of the Echiuridce (Fig. 137, p. 207} is very simple. It 

 consists of a ventral vessel running through the whole body and lying above the 

 ventral chord. Behind the mouth this vessel divides (like the ventral chord) into 

 2 branches which, embracing the mouth between them, pass along the 2 sides 

 of the prostomium to its extreme anterior end, where they unite (like the 2 limbs 

 of the oesophageal ring). A dorsal vessel arises from the point of junction, 

 which, running backwards, traverses the prostomium and then runs along the fore-gut 

 to the posterior end of the crop. Here it divides into 2 limbs, embracing the 

 intestine, and these enter the ventral vessel together. The dorsal vessel is thus con- 

 nected with the ventral vessel by 2 pairs of loops — one anterior, represented by the 

 lateral vessels of the prostomium, and one posterior, at the limit between the fore-gut 

 and the mid-gut. The dorsal vessel is not continued back over the fore-gut. 



Prosopygia. — A vascular system is wanting in the Priapulidw 

 among the Sipuiiculacea, and in all Bryozoa. In Sipunculus there are 

 two vessels, one dorsal and the other ventral, which accompany the 

 fore-gut. Both end posteriorly near the place where the proboscis 

 retractors are attached to the body wall. Anteriorly they enter a 

 sinus which lies in front of the brain at the base of the tentacle crown, 

 encircles the oral cavity, and is in communication with the inner cavities 

 of the tentacles. By the contraction of the dorsal and ventral vessels 

 the fluid they contain is forced into the circular sinus, and from it into 

 the tentacles, which consequently extend and expand. The elements 

 which float in the crelomic fluid are met with in the vascular fluid, 

 so that an open communication between the body cavity and the 

 vascular system probably exists. In Phoronis there is a closed blood- 



