240 ANNELIDA. 



habitats, invariably, in the hands of Dr. Williams, led to the following 

 results : The cephalic half, by this division of the body, does not lose 

 the power of locomotion. In a few days after the operation, it begins 

 to grow less active and vigorous in its movements, and the annulus at 

 the point of division begins to contract and wither ; in the course of a 

 few more hours it dies it mortifies away. This process of dissolution 

 creeps, in the direction of the head, from one segmental ring of the body 

 to another, until, finally, the cephalic remnant ceases to manifest any 

 signs of life. 



(629.) The tail-half immediately loses the power of advancing ; it 

 writhes on one spot, and that only in contact with some external body ; 

 its motions become excited, not voluntary, it never re-acquires the 

 power of swallowing earth. The process of decay begins much sooner 

 than in the cephalic half, and extends in the direction of the tail, im- 

 plicating one ring after another rapidly, until the whole perishes. 



(630.) The little lively Naides, although terricolous in their habits 

 like the Earthworm, are very dissimilar in organization. 



(631.) In Nais filiformis, so abundant in the freshwater pools of 

 this country, the anatomist is presented with a favourable opportunity 

 of resolving the problem of the circulation. A living specimen placed 

 between two slips of glass, from the perfect transparency of the integu- 

 ments, will exhibit to the eye in a perfect manner all the circulating 

 movements both of the vessels and of the blood. In Nais, the large 

 dorsal vessel (fig. 116, a) is first seen travelling wavingly along the 

 dorsum of the intestine as far as the heart, which corresponds in situa- 

 tion with the intestinal end of the oesophagus. This vessel is enveloped 

 by the glandular peritoneal layer of the intestine, while the coats of the 

 ventral vessel are clear and transparent : the dorsal vessel is endowed 

 with parietes of greater strength and density than the ventral. Each 

 of these vessels dilates into a fusiform heart (fig. 116, a', b'), situated on 

 either side of the oesophagus. These hearts, which are joined together 

 by transverse vessels, pulsate alternately and with exact regularity. 

 In the dorsal vessel the blood moves forwards from the tail as far as 

 the dorsal heart ; thence it descends into the ventral heart, by which it 

 is now propelled, chiefly in a backward direction, partly through the 

 main ventral trunk, and partly through the inferior intestinal. The 

 other portion of the blood conveyed by the great dorsal vessel into the 

 ventral heart (&') passes forwards as far as the head, where its moving 

 power is again reinforced by a cardiac dilatation, which now impels the 

 current from before backwards through a superior cesophageal trunk 

 into the dorsal heart (a'), by which organ the blood received from the 

 region of the ossophagus and coming from the head, as well as that 

 received from the great dorsal and coming from the tail, is urged down- 

 wards into the ventral heart, and thence, chiefly in the direction of the 

 tail, through the ventral and intestinal trunks (f,e); this latter there- 



