66 THE MEDICINAL LEECH 



taking care not to injure the alimentary canal, which lies close to the skin. 

 Carefully remove the integument from the alimentary canal and pin out the 

 flaps on each side. Note the dorsal blood-vessel lying over the aUmentary 

 canal in the mid-dorsal line. 



Note the nephridia, a pair occurring in each segment from the 6th to the 22nd. 

 Each nephridium hjis a closed btit porous funnel, the " testis-lobe," contained 

 in a cavity of the coelom above the testis, a much-coiled glandular main portion 

 with fine ciliated ducts and blood capillaries in its walls, and a muscular ciUated 

 bladder opening to the exterior by a nephridiopore. The inner end of the 

 " testis-lobe " probably corresponds to the nephrostome of the Earth-worm. 



A. The Alimentary System. 



The mouth is situated in the anterior sucker and is surrounded by the jaws. 

 These are arranged radially, one being median and dorsal, the other two ventro- 

 lateral. Each is a compressed muscular cushion with a chitinous surface pro- 

 duced into numerous teeth. The mouth leads into a muscular pharynx situated 

 in the 4th to 7th segments. It is connected by strong radiating muscles to the 

 body-wall. There are numerous unicellular glands in the wall of the pharjmx, 

 and the secretion of these glands prevents the coagulation of the blood. The 

 oesophagus, following the pharynx, is a very short tube leading into the exten- 

 sive crop. This consists of eleven pouches on each side, the first ten pairs of 

 these storing pockets being directed outwards and each corresponding to a 

 segment. The eleventh pair passes backwards parallel to the intestine and 

 reaches to the 24th segment. After giving off the eleventh pair the crop is 

 succeeded by the stomach, a small spherical dilatation, opening behind into the 

 intestine, a narrow straight tube passing into the somewhat dilated rectum, 

 which turns slightly upwards and opens at the anus in the last annulus of the 

 last segment. 



B. The Vascular System. 



This system is complex. It consists of a spongework of sinuses {i.e., irregular 

 channels with non-muscular walls) filled with a fluid which is not really blood, 

 but which apparently functions as such, namely, ccelomic fluid containing the 

 red blood-pigment, haemoglobin. The spongework completely fills the original 

 open space, the body-cavity or ccelome, which is consequently represented by 

 the complicated system of ccelomic sinuses, and these seemingly replace the 

 true blood-vessels of a proper blood system. The principal sinuses are : 



(a) The dorsal sinus, along the mid-dorsal line above the alimentary canal, 

 and connected behind with the ventral sinus by fine channels which 

 pass between the intestine and the last pair of storing pockets. 



(ft) The ventral sinus, under the alimentary canal. It surrounds and en- 

 closes the ventral nerve cord. 



(c) The lateral "blood-vessels," a pair of tubular channels along the sides 

 of the body. These are probably sinuses with muscular walls. They 

 are connected by cross channels, which pass beneath the ventral sinus. 



Make a drawing showing as much as possible of the alimentary system and 

 the vascular system. 



