annelida: — naiad-worms: teeth of leech. 635 



parence of their bodies, present peculiar facilities for Microscopic 

 examination, and especially for the study of the internal Circulation 

 of the red liquid commonly considered as Blood. There are here 

 no external Respiratory organs ; and the 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 dilatation, 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-off 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 vascular coils, hanging down in the Peri- 

 visceral cavity 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 Internal Gills. — The Naiad- worms 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 anterior. It has been generally believed that each Segment 

 continues to live as an entire Worm ; but it is asserted by Dr. T. 

 Williams 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 Dental ap- 

 paratus with which the Mouth is furnished is one of the most curious 

 among their points of minute structure ; and the common ' medi- 

 cinal' 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 semi- 

 circle, which is bordered by a row of eighty or ninety minute hard 

 and sharp Teeth ; whilst the straight border 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. 



