94 MORPHOLOGY OF INVERTEBRATE TYPES 



wall. Here the branches of the ventral vessel anastomose with 

 branches which return the oxidized blood to the dorsal vessel. 

 The dorsal vessel receives, therefore, mixed blood, the larger 

 part of which has been oxidized while some of it is venous. The 

 blood itself is red, but the color is contained in the plasm and 

 not in the corpuscles. 



Nervous system. The nervous system consists of a pair of 

 supracesophageal ganglia or brain and a ventral chain of paired 

 ganglia. The brain is situated in the third segment and gives 

 off anteriorly two pairs of nerves. Two lateral connectives unite 

 the brain with the subcesophageal or first pair of ventral ganglia. 

 The right and left elements of the nervous chain are so closely 

 connected with each other that their paired origin may be 

 recognized only on sections. The chain appears as a whitish 

 cord with a swelling in every segment, which marks the ganglia. 

 Each swelling gives off two pairs of nerves. There is also a pair 

 of dissepimental nerves given off by the ventral chain halfway 

 between the swellings of the chain. Thus every segment has 

 three pairs of nerves, a condition similar to that in Nereis and 

 Hirudo. Higher sense organs are absent. 



Reproductive system. Lumbricus terrestris is a herma- 

 phrodite like all Oligochaetes. The male reproductive organs 

 consist of two pairs of testes, two pairs of ciliated funnels, two 

 vasa deferentia, two seminal vesicles and three pairs of sper- 

 mothecae. The testes are very small and are situated in the tenth 

 and eleventh segments close to and above the nervous system. 

 They are inclosed in the seminal vesicles. The first seminal 

 vesicle which is situated in the tenth segment has two pairs of 

 spermothecse or lateral pouches and contains the first pair of 

 testes and the first pair of ciliated funnels. The second seminal 

 vesicle is situated in the eleventh segment, has one pair of large 

 spermothecae and contains the second pair of testes and the 

 second pair of ciliated funnels. The funnels perforate in both 

 segments the dissepimenta and open into the thin vasa defer- 

 entia. These two ducts are rather short, terminating in the two 



