SEGMENTAL INDIVIDUALS 115 



of connective tissue ; a layer of circular and then one of 

 longitudinal muscles ; and finally a thin layer of parietal 

 peritoneum. 



The main features of the circulatory system are a long 

 dorsal bloodvessel running the whole length of the body, 

 with another similar vessel running ventrally below the 

 digestive tube, and below this a third underlying the nerve- 

 cord. The different vessel-systems communicate in various 

 ways, but that which is of special interest is the linking 

 together of the dorsal and ventral vessels " by means of 

 five pairs of loops called hearts, situated in the seventh, 

 eighth, ninth, tenth and eleventh segments " (Shipley and 

 MacBride). 



Of equally important evolutionary significance is the 



Fia. 41. — The earthworm's central nervous system. Roman 

 numerals indicate the segments, m, mouth ; p, pharynx ; 

 eg, the cerebral ganglia ; nc, nerve-collar round pharynx ; vg, 

 first ventral ganglion of the chain which passes down through 

 succeeding segments ; n, nerves. (After Shipley and MacBride, 

 after Hesse.) 



nervous system of the worm. Lying above the pharynx 

 are two suprapharyngeal ganglia, from each of which passes 

 downwards and backwards a nerve-cord to unite to form 

 the first ventral ganglion. The pharynx is thus surrounded 

 by a nerve-collar or commissure connected below with the 

 ventral nerve-cord, which is formed by the linking together 

 of the nerve-ganglia of all segments behind the fourth ; 

 one ganglionic swelling being situated in each segment 

 (Fig. 41). 



In every segment except the last and the first three 

 there is a pair of excretory organs, or " nephridia." We 

 are not concerned with their structure, but have to note 

 the repetition here exhibited. 



