238 MANUAL OF ELEMENTARY ZOOLOGY 



corpuscles. One of these systems is the true blood-vascular 

 system : its principal vessels are two lateral trunks, which 

 unite before ?nd behind, and are connected also by a net- 

 work of capillaries. There are no hearts, but the lateral 

 vessels are contractile. The other vascular system consists 

 of a dorsal and a ventral longitudinal sinus, which are also 

 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 ccelom, 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 botry.oidal tubes are also in com- 

 munication with the sinus system. It is doubtful whether 

 the capillaries of the true blood system are connected with 

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

 earthworm and Nereis. A small brain, above and before 

 the pharynx, is connected by a pair of very short circum- 

 pharyngeal commissures with a ventral cord which carries at 

 wide intervals twenty-three round 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 com- 

 missures 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-2oth seg- 

 ments, with on each side a common vas deferens, which is 

 coiled as an "epididymis" in the 10th segment, 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 segment, in which its 

 short oviducts open by a common vagina. The eggs are 

 laid in cocoons secreted by 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. 



Segmented, ccelomate animals, with a thin cuticle, a 

 Annelida closed blood-vascular system, and nephridia 

 and a nervous system like those of the earth- 

 worm, are known as Annelida. The earthworm, Nereis 

 the lugworm {Arenicola), often used for bait, and leeches' 

 belong to this group. 



