THE ANATOMY. VASCULAR SYSTEM 73 



it appears to be due to the fact that the commissural vessels of the posterior segments 

 have given rise to the integumental blood-plexus, and have become largely lost in 

 the process ; there are, as I shall point out later, the beginnings of this plexus to 

 be seen in the Tubificidae ; it attains to its highest development in the Oligochaeta 

 terricola ; but in the embryo Lumbricus, as VEJDOVSKY has shown (see woodcut, fig. 19), 

 each segment has a pair of commissural vessels. The number and position of the 

 hearts in the earthworms varies very considerably ; it is rare, however, to find them 

 extending behind the thirteenth segment. As a rule the last three or four, often two 

 of these, are connected either with both the dorsal and supra-intestinal vessels or with 

 the latter alone ; this kind of connexion, however, does not occur in the Lumbricidae, 

 and apparently not in the Geoscolicidae ; it occurs only in the Megascolicidae and the 

 Eudrilidae among the Megadrili ; the Moniligastridae agree in this character with the 

 Geoscolicidae and the Lumbricidae. As a rule the intestinal hearts are much more 

 dilated than the others, and they show in a more pronounced fashion the monilifonn 

 character which these organs often exhibit ; this is really due to the presence of valves 

 along the course of the commissural vessels, which allow the blood to flow in one 

 direction only. 



4. Peripheral circulation. 



In addition to the longitudinal trunks and to the commissural vessels which 

 unite them, there is in all Oligochaeta a system of smaller vessels, which form 

 plexuses and may be termed capillaries ; it is convenient to divide these capillary 

 networks into two series, the integumental and the intestinal. The former includes 

 all the capillaries which ramify in the thickness of the body-wall, the septa, and 

 in fact all the organs of the body except the alimentary system ; the reasonableness 

 of making this apparently artificial division is shown by the fact that in the lowest 

 Oligochaeta there is no capillary system except that which supplies the intestinal 

 walls, and that the two systems are quite distinct, being only indirectly connected 

 in the higher forms. In the Aphaneura, the Enchytraeidae, and the Naidomorpha, 

 the intestinal system of capillaries is the only one that is present. In the two 

 former groups the dorsal vessel loses itself in this plexus ; it seems a little doubtful 

 whether in the adults of any of these worms there really exists, as has been 

 described, a blood-holding space surrounding the gut ; when the capillaries are 

 gorged with blood, there would naturally be a tendency to the obliteration of the 

 boundaries of the meshes of the network which would of course produce the 

 impression of a continuous sinus. As to Aeolosoma, however, the figures of 

 VEJDOVSKY show plainly that there is a plexus and not a sinus; in the Enchytraeidae 



L 



