OLIGOCHAETA. 



495 



the body -form, the ventral nervous system, etc., the variations 

 in structure in the excretory and generative organs are most 

 remarkable. 



The setae are small chitinous rods projecting from the body- wall. 

 Their lower ends are embedded in small ectodermal pits, tne cells 

 of which secrete them. They are absent in the Discodrilidae and 

 in the genus Anachaeta, otherwise they are present to the number of 

 eight or a larger number on most of the segments. When there 

 are eight they are usually arranged in four groups of two on the 

 ventral side, but the number and arrangement presents great variety, 

 and in some forms (Perichaetidae) there is a continuous circle of 

 them round each segment (Fig. 411). The usual form of seta is 

 that of anyj but they vary somewhat, as is shown by the accompanying 



FIG. 404. 1, 2, Penial setae of Acanthodrilus georgianus. 3, Spirosperma ferox. It, Hyodrilus 

 coccineus. 5, Lophochaeta ignota. 6, Tubifex rivnlorum. 7, 8, Nais. 9, Bohemilla comata. 

 (From Beddard.) 



diagrams. The setae are retractile and protractile by special muscles 

 attached to the walls of the setigerous sacs (Fig. 405). They are 

 organs which assist in locomotion. 



Branchiae are found in a few forms, e.g. Chaetobranchus semperi, 

 Branchiura Soicerbyi, and Hesperodrilus branchiatus. 



The body-wall consists of a cuticle, a layer of ectoderm, an outer 

 circular and an inner longitudinal muscular layer. The ectoderm 

 consists of cells, many of which are glandular. It also contains 

 tactile sense organs in the form of papillae, and organs which resemble 

 taste-buds; and in the aquatic forms many of the ectoderm cells 

 carry sensory hairs. The gland-cells are especially well-developed in 

 the clitellum. 



