74 . OLIGOCHAETA 



a sinus has been generally asserted to exist ; but MICHAELSEN figures in Stercutus, 

 for example, appearances which are much more in accord with the assumption that 

 there is really a network : he speaks, however, in this case of a ' Darmblutsinus.' 

 The same word is used in his revision of the family, and therefore containing his 

 latest opinions on the matter, with which also VEJDOVSKY agrees. The existence 

 of this sinus, if real, is of interest in connexion with the perienteric sinus in certain 

 Chaetopods (for example Fabricia) ; the fact that in Aeolosoma and the Enchytraeidae 

 the dorsal vessel arises from this sinus becomes additionally interesting from SALENSKY'S 

 discovery that in Terebdla, etc., the embryo has a dorsal vessel similarly connected 

 with, indeed arising from, a perienteric blood-sinus. As to the Oligochaeta, embryology 

 does not seem to indicate that this sinus is primitive ; in Rhynchelmis VEJDOVSKY 

 describes the dorsal vessel as arising from the perienteric plexus (not sinus). It 

 will be noted, however, that the limited extent of the dorsal vessel and its origin 

 out of the perienteric vessels is an embryonic character in the Aphaneura and 

 Enchytraeidae. Whatever may be the case with the lowest Oligochaeta, it is certain 

 that in the Naidomorpha and in all the groups above them, there is not a plexus 

 but a network of capillaries in the intestinal walls ; this has been figured by STOLC 

 and others in the Naidomorpha, arid by the same and also by others in the 

 Tubificidae ; in these worms the network is fed from the dorsal vessel, and the 

 blood returns into the ventral vessel ; the network at any rate has connections 

 with both vessels. Among earthworms Sparganophilus has been said to possess 

 a sinus by BENHAM ; I confess to being unwilling to accept this statement, for 

 the reasons already stated, i. e. the difficulty of proving that the supposed sinus 

 is not really a plexus with largely obliterated boundaries. The intestinal network 

 of the higher Oligochaeta is described below in the genus Megascolex, in which it 

 has been carefully studied by BOURNE. 



The integumental blood plexus shows a gradual increase in complexity as we 

 pass from the higher to the lower forms. It is, as already stated, entirely absent 

 in the Enchytraeidae and the Aphaneura. The first traces are to be found in 

 Limnodrilus, where the lateral commissural vessels give off (in Limnodrilus hoff- 

 nieisteri at any rate) a bunch of small vessels which seem to end caecally in the 

 skin. In other Tubificidae the integumental system is much more developed ; in 

 Ilyodrilus, for example, where the circulation has been carefully studied by S TOLC, there 

 is a complex network in the integument ; it appears from his figures (the text being 

 in the Bohemian language is inaccessible to me) that in each segment of the body 

 behind the first a special pair of vessels is given off from the dorsal vessel which 

 supply the plexus, and that branches of the commissural vessels receive the blood from 



