HIRUDO MEDICINALIS 183 



connected before and behind and by means of a capillary system. 

 The walls of this system are thinner than those of the true blood 

 vessels. They represent the coelom, as may be seen from the fact 

 that the ventral sinus encloses the nerve cord and communicates 

 with capsules around the ovaries and testes. The botryoidal 

 tubes are also in communication with the sinus system. It is said 

 that the capillaries of the true blood system are connected with 

 it. The nervous system is of the same type as those of the earth- 

 worm and Nereis. A small brain, above and before the pharynx, 

 is connected by a pair of very short circumpharyngeal commissures 

 with a ventral cord which carries at wide intervals twenty-three 

 ganglia. Of these the first or subpharyngeal and the last each 

 represent several fused. Nerves are given off from the brain 

 and the ganglia. The commissures which unite the ganglia of the 

 ventral cord are seen in section to be double, with a slender 

 median strand. 



The animal is hermaphrodite. There are nine pairs of testes, 

 enclosed in spherical capsules in the I2th-20th somites, with on 

 each side a common vas deferens, which is coiled as an ' epi- 

 didymis ' in the loth somite, where the vasa deferentia join to 

 open on a muscular, protrusible penis, surrounded at its base by 

 a ' prostate ' gland. The single pair of ovaries lies in the nth 

 somite, in which its short oviducts open by a common vagina. 

 The eggs are laid in cocoons secreted by ' clitellar ' glands in the 

 skin of the ioth-i2th segments and placed in holes made in the 

 banks, above water. The young resemble the parents and feed 

 at first on the juices of water insects and the like. 



ANNELIDA 



The phylum AnneUda, to which the creatures described in 

 this chapter belong, is very difhcult to define, since its most 

 distinctive features are not possessed by all its members. Annelids 

 are segmented animals with one preoral segment, the prostomium, 

 but all their other universal characteristics they share with other 

 groups ; they are triploblastic and coelomate ; the nervous 

 system consists of a double ventral cord, usually with a pair of 

 gangha in each segment, and of a pair of commissures surrounding 

 the gut and leading to a pair of preoral gangha ; they have 

 nephridia and ccelomoducts ; the larva if present is a trochosphere. 

 Negative characters of importance are the absence of any 



