ANNULOSA, OK WORMS. 203 



rivers and ponds, living chiefly amidst the mud at the bottom, and espe- 

 cially among the roots of aquatic plants. Being blood-red in color, they 

 give to the surface of the mud, when they protrude themselves from it 

 in large numbers and keep the protruded portion of their bodies in con- 

 stant undulation, a very peculiar appearance; but if disturbed, they with- 

 draw themselves suddenly and completely. These Worms, from the 

 extreme transparency of their bodies, present peculiar facilities for Mi- 

 croscopic examination, and especially for the study of the internal circu- 

 lation of the red liquid commonly considered as blood. There are here 

 no external respiratory organs; and the thinness of the general integu- 

 ment 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 con- 

 tractile 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, the stream branches- oif 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 perivisceral cavity that con- 

 tains the corpusculated liquid representing the true blood, which seems 

 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 a complete worm; but it is asserted by Dr. T. Wil- 

 liams 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 f nurses ' of the eggs which both in- 

 clude. In the Leech tribe, the dental apparatus with which the mouth 

 is furnished, is one of the most curious among their points of minute 

 structure; and the common ' medicinal 9 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, radi- 

 ating from a common centre. If the mouth of a leech be examined with 

 a hand-magnifier, or even with the naked eye, it will be seen to be a tri- 

 angular 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 bor- 

 dered by a row of eighty or ninety minute hard and sharp teeth; whilst 

 the straight border of the semicircle is imbedded in the muscular sub- 

 stance of the disk, by the action of which it is made to move back- 

 wards 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. l 



1 Among the more recent sources of information as to the Anatomy and Phy- 

 siology of the Annelids, the following may be specially mentioned: The " His- 

 toire Naturelle des Anneles Marins et d'Eau douce " of M. de Quatrefages, forming 

 part of the " Suites a Buff on; " the successive admirable Monographs of the late 

 Prof. Ed, Claparede, "Recherches Anatomiques sur les Annehdes, Turbellaries, 



