PHYLUM ANNELIDA. SEGMENTED WORMS 



173 



grooves extending around the body. At the 

 anterior end a fleshy lobe, the prostomium 

 (Fig. 94), projects over the mouth; this is 

 not here considered a true segment, although 

 some authors regard the prostomium as the 

 first true segment. It is customar)' to number 

 the segments, beginning at the anterior end, 

 since both external and internal structures 

 bear a constant relation to them. Segments 

 31 or 32 to 37 are swollen in mature worms, 

 forming a saddle-shaped enlargement, the 

 clitellum, of use during reproduction. Every 

 segment, except the first and last, bears 4 

 pairs of chitinous bristles, the setae (Fig. 

 92); these may be moved by retractor and 

 protractor muscles and are renewed if lost. 

 The setae on segment 36, in mature worms, 

 are modified for reproductive purposes. 



The body is covered by a thin transparent 

 cuticle secreted by the cells lying just be- 

 neath it. The cuticle protects the body from 

 physical and chemical injury; it contains nu- 

 merous pores to allow the secretions from 

 unicellular glands to pass through; and it is 

 marked with fine striations, causing the 

 surface to appear iridescent. 



A number of external openings of various 

 sizes allow the entrance of food into the 

 body, and the exit of feces, excretory prod- 

 ucts, reproductive cells, etc. ( 1 ) The mouth 

 is a crescentic opening situated in the ven- 

 tral half of the first segment (Fig. 94); it 

 is overhung by the prostomium. (2) The 

 oval anal opening lies in the last segment. 

 (3) The openings of the sperm ducts or 

 vasa deferentia are situated on each side of 

 segment 15 (Fig. 91); they have swollen 

 lips, and a slight ridge extends posterioriy 

 from them to the clitellum. (4) The open- 

 ings of the oviducts are small round pores, 

 one on either side of segment 14; eggs pass 

 out of the body through them. (5) The 

 openings of the seminal receptacles appear 

 as 2 pairs of minute pores concealed within 

 the grooves which separate segments 9 and 

 10, and 10 and 11. (6) A pair of nephridio- 

 pores (Fig. 92), the external apertures of 

 the excretory organs, open on every seg- 



ment except the first 3 and the last. They 

 are usually situated immediately anterior to 

 the outer setae of the inner pair. (7) The 

 body cavity or coelom communicates with 

 the exterior by means of dorsal pores. One 

 of these is located in the middorsal line at 

 the anterior edge of each segment from 8 

 or 9 to the posterior end of the body. 



Internal anatomy 



If a specimen is cut open from the an- 

 terior to the posterior end by an incision 

 passing through the body wall, a general 

 view of the internal structures (Fig. 91 ) may 

 be obtained. The body is essentially a double 

 tube (Fig. 91); the body wall constituting 

 the outer, and the straight digestive tract, 

 the inner; between the two is a cavity, the 

 coelom. The external segmentation corre- 

 sponds to an internal division of the coe- 

 lomic cavity into compartments by means 

 of partitions called septa, which lie beneath 

 the grooves (Fig. 91). The digestive tract 

 passes through the center of the body and is 

 suspended in the coelom by the partitions. 

 Septa are absent between segments 1 and 

 2 and incomplete between segments 3 and 

 4, and 17 and 18. The walls of the coelom 

 are lined with an epithelium termed the 

 peritoneum (Fig. 92), which is derived 

 from the mesoderm. 



The coelomic cavity is filled with a 

 colorless fluid which flows from one com- 

 partment to another when the body of 

 the worm contracts, thus producing a sort 

 of circulation. This is possible since a 

 large opening is present in the median ven- 

 tral part of each septum. In segments 9 to 

 16 are the reproductive organs; running 

 along the upper surface of the digestive 

 tract is the dorsal blood vessel; and just 

 beneath it lie the ventral blood vessels and 

 nerve cord. The body wall contains 2 layers 

 of muscles. The outer layer lies beneath the 

 epidermis and consists of circular muscle 

 tissue. The muscle fibers are long and spin- 

 dle-shapedj when they contract, the diam- 



