148 INVERTEBRATE ZOOLOGY 



The central nervous system consists of a ventral chain with a 

 ganglion in each segment. This chain, which is really composed 

 of two fused cords, frequently shows its double nature in its 

 ladder-like appearance, and in cross-section the arrangement of 

 cells and fibers give still further evidence. In the anterior region 

 of the body, the two cords separate to pass around the pharynx 

 on the dorsal surface of which occurs the largest ganglion, called 

 the suprapharyngeal ganglion or brain. 



Excretion is by means of metanephridia in the adults and 

 protonephridia in larvae of forms which have the trochophore. 

 Characteristically, each somite is provided with a pair of meta- 

 nephridia the funnel or nephrostome of each of which is attached 

 to the posterior septum while the tubule penetrates the septum 

 and passes into the cavity of the adjacent somite before opening 

 to the outside through the nephridiopore. Modified nephridial 

 ducts (Fig. 47) are frequently utilized for the discharge of the 

 germ cells. Primitively, separate ducts called ciliated grooves 

 or coelomoducts serve for the discharge of the germ cells, but 

 these frequently become associated with the nephridia, the tubules 

 of which then becomes the gonoducts. 



Embryology. — Annelid eggs undergo cleavage of determinate 

 type which results in the formation of a definite pattern in the 

 arrangement of the blastomeres. From polar view the cells are 

 arranged in the form of a cross. So characteristic is this pattern 

 (Fig. 10) that the term annelidan cross has been applied to 

 it. Later development results in the formation of a trochophore 

 except in fresh-water and terrestrial forms. The structure and 

 transformation of this larva will be described later. Asexual 

 reproduction is not uncommon. In some forms, there is but 

 slight specialization of the individual somites. This condition, 

 known as homonomous metamerism, facilitates asexual repro- 

 duction and frequently leads to the formation of a chain of 

 individuals through the differentiation of the metameres of a 

 single worm. 



Subclass POLYCHAETA 



The sexes are separate in the polychaetes. Usually, the 

 anterior somites are rather highly specialized to form a distinct 

 head bearing eyes and tactile organs. The somites have out- 

 growths from their lateral margins which are known as parapodia 

 and serve as rudimentary appendages. Each parapodium is sup- 



