Annelidcs, 327 



occurring in the medicinal leech.* This is made out in the following 

 way. Those parts of the glandular system which will be described 

 as the loop-shaped glands, are considered to be blood-vessels coming 

 off from the lateral trunks ; these carry the blood to the walls of the 

 mucous sacs, where it is brought into contact with the aerated water 

 contained in the sacs, and from which it is again conveyed to the la- 

 teral trunk by means of a small communicating vein. 



Sensation, — Of the five senses which are common to animals of 

 the higher classes, that of sight is the most perfect in the horse- 

 leech. The eyes of this creature are ten in number, and are placed 

 on the dorsal surface of the anterior sucker, (fig. 3) : six. of them are 

 arranged in a curved form near the margin of the disk, three on each 

 side of the median line, with a slight space between them ; behind 

 these, and at a little distance, we have two others, and at a still great- 

 er distance of a line and a half, are the remaining two, which are 

 much larger than any of the preceding. They are hardly perceptible 

 to the naked eye, but when examined microscopically with a low pow- 

 er, they appear as so many black dots, and with a power of 100 linear 

 the anterior part of the eye is seen not to be black, but transparent ; 

 the posterior part is covered with a black pigment, which corresponds 

 to the choroid in the eyes of other animals (fig. 7, 8). The optic nerve 

 enters at the back, and the whole globe of the eye seems to be nothing 

 more than the dilated or bulbous extremity of the optic nerve. Miil- 

 ler, who has examined the eyes of the Annelida with the greatest care, 

 finds some of them to be spherical, but in the leech tribe they appear 

 to be rather of an oval figure. The sense of touch is likewise pre- 

 sent to a considerable extent in these animals ; but they appear to be 

 quite deficient in that of hearing. 



Nervous System. — The nervous system (as in the other members of 

 the class Annelida), may be said to consist of a bilobed ganglion or 

 brain, situate near the head, and a cord composed of two columns, 

 which extends from the brain to the caudal extremity, all lying on the 

 abdominal surface of the body, upon the alimentary canal (fig. 1). The 

 brain is situated about two lines from the mouth, its anterior portion be- 

 ing circular, whilst its posterior is more of a triangular figure -, both are 

 firmly imbedded in the thick muscular tissue appended to the mouth 



* Moquin Tandon, ' Monographie de la Famille des Himdinees.' Duges, ' An- 

 iiales des Sciences Naturelles,' tome xv. 



