538 ANNULOSA, OK WORMS. 



thinness of the general integument appears to supply all needful 

 facility for the aeration of the fluids. One large vascular trunk 

 (dorsal) may be seen lying above the intestinal canal, and another 

 (ventral) beneath it ; and each of these enters a contractile dilata- 

 tion, or heart-like organ, situated just behind the head. The 

 fluid moves forwards in the dorsal trunk as far as the heart, 

 which it enters and dilates; and when this contracts, it propels 

 the fluid partly to the head, and partly to the ventral heart, which 

 is distended by it. The ventral heart, contracting in its turn, 

 sends the blood backwards along the ventral trunk to the tail, 

 whence it passes towards the head as before. In this circulation, 

 it branches oflT from each of the principal trunks into numerous 

 vessels proceeding to different parts of the body, which then 

 return into the other trunk ; and there is a peculiar set of vascu- 

 lar coils, hanging down in the perigastric space that contains the 

 corpusculated liquid representing the true blood, which seem 

 specially destined to convey to it the aerating influence received 

 by the red fluid in its circuit, thus acting (so to speak) like inter- 

 nal gills. The Naid-wonns have been observed to undergo 

 spontaneous division during the summer months, a new head 

 and its organs being formed for the posterior segment behind 

 the line of constriction, before its separation from the interior. 

 It has been generally believed that each segment continues to 

 live as an entire worm ; but Dr. T. Williams has lately asserted, 

 that from the time when the division occurs, neither half takes 

 in any more food, and that the two segments only retain vitality 

 enough to enable them to be (as it were) the "nurses" of the 

 eggs which both include. In the Leech tribe, the apparatus of 

 teeth with which the mouth is furnished, is one of the most 

 curious among their points of minute structure ; and the common 

 "medicinal" leech affords one of the most interesting examples 

 of it. What is commonly termed the "bite" of the leech, is 

 really a saw cut, or rather a combination of three saw cuts, 

 radiating from a common centre. If the mouth of the leech be 

 examined with a hand-magnifier, or even with the naked eye, it 

 will be seen to be a triangular aperture in the midst of a sucking 

 disk ; and on turning back the lips of that aperture, three little 

 white ridges are brought into view. Each of these is the convex 

 edge of a horny semicircle, which is bordered by a row of eighty 

 or ninety minute hard and sharp teeth ; whilst the straight bor- 

 der of the semicircle is imbedded in the muscular substance of 

 the disk, by the action of which it is made to move backwards 

 and forwards in a saw-like manner, so that the teeth are enabled 

 to cut into the skin to which the suctorial disk has affixed itself. 



