PHYLUM ANNELIDA 217 



XXXI or XXXII to XX XVII uiv swollen in matuiv \\orms, 

 forming a saddle-shaped enlargement, the ditdlum, of use during 

 reproduction. Every segment except the first and last bears 

 four pairs of /-shaped chitinous bristles, the seta, situated as 

 indicated in Figure 153, set; these may be moved by retractor 

 and protractor muscles, and are renewed if lost. The setae on 

 somite XXVI are in mature worms modified for reproductive 

 purposes. 



The body is covered by a thin, transparent cuticle (Fig. 153, 

 cut) secreted by the cells lying just beneath it. The cuticle 

 protects the body from physical and chemical injury; it con- 

 tains numerous pores to allow the secretions from unicellular 

 glands to pass through, and is marked with fine stria, causing 

 the surface to appear iridescent. 



A number of external openings of various sizes allow the en- 

 trance of food into the body, and the exit of fasces, excretory 

 products, reproductive cells, etc. (i) The mouth is a crescentic 

 opening situated in the ventral half of the first somite (Fig. 156, 

 5) ; it is overhung by the prostomium (Fig. 156, i). (2) The oval 

 anal aperture lies in the last somite. (3) The openings of the 

 sperm ducts or vasa deferentia are situated one on either side of 

 somite XV. They have swollen lips; a slight ridge extends 

 posteriorly from them to the clitellum. (4) The openings of the 

 oviducts are small, round pores one on either side of somite XIV; 

 eggs pass out of the body through them. (5) The openings of 

 the seminal receptacles appear as two pairs of minute pores con- 

 cealed within the grooves which separate somites IX and X, 

 and X and XI. (6) A pair of nephridiopores (Fig. 153, e.vt. ncph.}, 

 the external apertures of the excretory organs, open on every 

 somite except the first three and the last. They are usually 

 situated immediately anterior to the outer seta of the inner pair. 

 (7) The body-cavity or ccelom (Fig. 153, ccel.) communicates 

 with the exterior by means of dorsal pores. One of these is lo- 

 cated in the mid-dorsal line at the anterior edge of each somite 

 from VIII or IX to the posterior end of the body. 



