582 COMPARATIVE ANATOMY CHAP. 



the trunk, receives blood from the proboscis through two lateral vessels 

 or vascular plexuses within the two vascular folds of the collar, these 

 vessels or plexuses usually uniting in the ventral median line at the 

 posterior end of the collar region. The dorsal and the ventral 

 vessels of the trunk have muscular walls, which, however, do not 

 properly belong to them, but are borrowed from the apposed walls of 

 the mesenterial portions of the coelomic sacs. In the proboscis, the 

 blood vascular system, by increase of its surface towards the probosci- 

 dal coelom, to right and left of the basal complex of organs, gives 

 rise to the so-called proboseidal gill or glomerulus. 



Special. The finer details cannot be entered upon. 



1. Vessels of the trunk. While the dorsal mesentery is retained in the 

 abdominal part of the body (as already noted, the ventral mesentery persists 

 throughout the whole trunk) it may disappear in the anterior trunk region with the 

 exception of the part which contains the dorsal longitudinal vessel. The muscles 

 of the vascular trunks are transverse or circular muscles, 'and, in part at least, 

 contimiations on to the mesenteries of the circular musculature of the body. The 

 ventral vascular trunk of B. Kowalevskii is provided, not with a transverse, but 

 with a longitudinal musculature. The musculature (which is yielded by the 

 mesenterial endothelium) always lies on the side of the limiting membrane 

 away from the lumen of the vessel, and facing the body cavity. 



2. The dorsal vessel of the collar is the direct continuation of the dorsal vessel 

 of the trunk. It runs between the two perilmemal cavities, from whose walls it 

 borrows its musculature. Passing out again from between these cavities, it loses 

 its musculature and opens, in the proboscis, into a blood sinus, the basal sinus of 

 the proboscis. This is a space left between various heterogeneous organs, the 

 proboscis pore, the diverticulum of the intestine, the posterior tip of the "heart 

 vesicle," the epithelium of the neck of the proboscis. 



This basal sinus communicates, on the one side, with the capillary network in 

 the wall of the proboscis, and on the other, through a narrow slit, with the central 

 blood sinus of the proboscis which lies in front of it. 



3. The central blood sinus of the proboscis (Fig. 464, 9) is a space in that limit- 

 ing membrane which separates the " heart vesicle" (dorsally) from the proboseidal 

 diverticulum of the buccal cavity (ventrally). It has no musculature of its own. 

 This, however, is supplied by the ventral transverse musculature of the "heart 

 vesicle " which lies above it. In Schizocardium, the central blood sinus is con- 

 tinued in a peculiar manner, which cannot here be further described, on to the 

 two "auricles" of the "heart vesicle." 



4. The proboseidal glomerulus (Fig. 464, 10) consists of two lateral principal 

 portions and a dorsal connecting piece. Each of the principal portions has the 

 form of a unilaminar honeycomb, with deep cells. The base of the comb is formed 

 by the right or left lateral walls of the basal complex of proboseidal organs, i.e. of 

 the ' ' heart vesicle " and the proboseidal diverticulum of the intestine. The apertures 

 of the single "cells," however, are turned towards the proboseidal coelom. The 

 walls of the cells are formed by folds of the visceral endothelium of the proboseidal 

 ccelom. They are hollow, and the cavities are blood sinuses, which open into a 

 common cleft-like sinus in the base of the comb. This latter, again, communicates, 

 by means of a slit-like transverse aperture, between the "heart vesicle" and the 

 proboseidal diverticulum, with the central sinus of the proboscis (Fig. 464, 9). 

 Posteriorly, each lateral principal part of the glomerulus becomes simpler and 



