184 



SYSTEMATIC ZOOLOGY. 



dorsal blood-vessel (d) in which the blood flows forward, 

 and a ventral vessel (v) in which it flows in the opposite 

 direction with pairs of vessels (cv), segmen tally arranged, 

 connecting the two. The nervous system is composed 



FIG. 28. Diagram of trunk somites of an Annelid, am, muscles of aciculi; c, 

 coelom; cm, circular body muscles; cv, circular blood-vessels ; d, dorsal blood- 

 vessel; i, intestine; Im, longitudinal body muscles; m, mesentery ; n, ventral 

 nerve cord; na, nephridium; ne, no, parts of parapodium; s, septum; sp, 

 splanchnopleure ; t, typhlosole. 



of a centre, or brain, above and in front of the mouth, 

 from which a pair of nerve cords (cesophageal commissures) 

 pass on either side of the oesophagus to connect with a 

 ventral nerve cord (n) which extends the length of the 

 body beneath the alimentary canal, and which bears 

 nervous enlargements, or ganglia, one pair in each somite. 

 Thus it will be seen that the brain is dorsal, the rest 

 of the nervous system ventral, to the alimentary canal; 

 in other words, the digestive tract passes through the 

 nervous system. The excretory system consists of a 

 system of organs called nephridia (na). Typically there 

 is a pair of these to each segment. Each consists of a 

 tube opening at one end by a funnel-shaped expansion, 



