THE ANATOMY. NERVOUS SYSTEM 19 



the body-cavity, are in connexion with the epidermis. In this worm — and it is 

 quite unique among the Oligochaeta — ^the entire central nervous system is confined to 

 the cerebral ganglia ; the ventral nerve cord appears to be entirely absent, except in 

 Aeolosoma tenebrarum, where it exists in a rudimentary form in the shape of a few 

 scattered cells. 



In all other Oligochaeta there are not only the cerebral ganglia connected by the 

 circumoesphageal commissures with the ventral chain, but there is in addition a system 

 of small ganglia arising from the cerebral ganglia, and concerned with the nerve-supply 

 of the anterior section of the alimentary canal ; in some forms also there is a ' lateral line 

 system.' These various parts of the central nervous system may now be considered 

 in detail. 



Cerebral Ganglia. These lie further forward in the lower than in the higher 

 Oligochaeta ; in the Tubificidae and the lower forms generally they are situated in the 

 fii-st segment ; in the earthworms, almost without exception, they have moved back 

 to the third segment ; but the development shows that the former is the primitive 

 position ; they are moved back in the earthworms by the invagination of the stomo- 

 daeum. In most, if not all, earthworms the cerebral ganglia mark the junction of the 

 buccal cavity and the pharynx. Why it should be so is mysterious, but it is a fact that 

 in the more highly organised Oligochaeta the brain is smaller and simpler than in the 

 lower forms. In the latter — in the Tubificidae and Naidomorpha for example — the 

 brain is not only relatively large, but it is provided with accessory lateral lobes, and 

 is often prolonged posteriorly into posterior lobes. The form of the brain in these worms 

 is often highly characteristic of the genus or species. Some references to the particular 

 form of the brain will be found in the systematic part of this work. In Phreoryctes 

 the brain has the simple bilobed character that is characteristic of the higher Oligochaeta 

 to which this worm is related. The Lumbriculidae also have a simple brain. Among 

 the Tubificidae there is often an impaired anterior median prolongation of the brain, 

 which sometimes comes to be detached and remains in connexion with the brain by 

 a nerve-cord. The formation of this anterior ganglion is highly suggestive of the buccal 

 ganglia in the Mollusca. Special muscles are often attached to the brain in the lower 

 Oligochaeta, which are apt to be confounded with nerves derived from it ^. The cerebral 

 ganglia give off a number of peripherally running nerves. 



The simplest arrangement of these again is found in the higher Oligochaeta — a fact 



which must, as it appears to me, be taken into consideration in fixing their position with 



respect to the so-called 'lower' forms. Vfjdovsky only finds one pair in the genera 



Lumbricus and Criodrilus ; Rosa figures an identical disposition for Hormogaster, and 



' They are attached of course to the connective tissue-sheath. 



D 2 



