72 



OLIGOCHAETA 



Fig. 18. 



' hearts.' In Tuhifex rivulonom, for example, the commissural vessels of segment viii 

 are greatly dilated and contractile. In Limnodrilus and Clitelli there are two such 

 contractile circles, but they are not always larger than the unmodified perivisceral 

 trunks. In other Tubificids a larger number of the anterior periviscerals are modified 

 into contractile hearts. Further details will be found under description of Tubificidae. 



In the Naidomorpha the direct connexions of the dorsal 

 and ventral vessels only exist in a few of the anterior 

 segments ; this also is the case with the Enchytraeidae 

 and the Aphaneura. The smallest number of commissural 

 vessels occurs in Pristina equiseta, where Bourne only 

 found one. The fact that in the newly-formed bud of 

 Uncinais littoralis there are commissural vessels in all 

 the segments, most of which become afterwards lost, is an 

 indication that the disappearance of all but the few anterior 

 commissural vessels in the Naidomorpha and Enchytraeidae 

 is due rather to degeneration than to the retention of any 

 primitive character. In some Tubificids there is another 

 class of commissural vessel in the anterior segments of 

 the body ; one or more of the hearts instead of forming the 

 dorsal and the ventral vessels unite the supra-intestinal 

 with the ventral. The name ' intestinal hearts,' introduced 

 by Pekriee for the corresponding structures in earthworms, 

 may be applied to these. It is difficult to determine whether 

 or not these trunks are serially homologous with the 

 remaining hearts and with the peri-intestinal vessels of 

 the segments in the posterior region of the body. 



In Bothrioneuron, Lophochaeta, and Phreodrilus, the 

 intestinal hearts occur in segments posterior to those which 

 contain the dorsal hearts ; but in Branchiura the eighth 

 segment contains two perivisceral vessels, of which one 

 seems to be the equivalent of the intestinal, the other of 

 the dorsal heart. 



In the higher Oligochaeta the intestinal hearts are 

 connected in many cases with both of the dorsally running blood-vessels ; so that one 

 connection may be regarded as secondary. In these Oligochaeta, in the earthworms 

 in fact, the commissural vessels are confined to the anterior segments of the body; 

 but this specialization is not due, as it may be in the Naidomorpha, to degeneration ; 



LOPHOCHAETA. VASCULAR 



SYSTEM OP SOME OE THE 

 ANTERIOR SEGMENTS. 

 (After Stole.) 



I. Ventral vessel. 2-6. Vessels 

 joining intestinal network. 7. Slen- 

 der posterior part of ventral vessel 

 just after it joins. 12. Sub-intestinal 

 vesseL 9, 13. Intestinal hearts. 15, 

 16. Conunissural vessels. 17. Dorsal 

 vessel. 



