ANNELIDA. 65 



spermatophores (composed of sperms derived from another indi- 

 vidual) from the spermothecae. The egg-capsule or cocoon is 

 passed forwards over the front end of the worm, and cylindrical 

 to begin with immediately becomes spindle-shaped by closure of 

 its ends. The spermatophores break down within the capsule, 

 and the liberated sperms fertilize the ova, polar cells having pre- 

 viously been formed. 



7. Muscular System. As in the Fluke, most of the muscle is 

 found in the body-wall. Underlying the skin is a continuous 

 circular layer in which the pigment that gives the dorsal surface 

 its characteristic tint is imbedded. Delicate bands, the protractor 

 muscles of the setae, pass from this layer to the inner ends of the 

 setigerous sacs. Beneath the circular layer is a longitudinal layer 

 of about the same thickness, subdivided into longitudinal bands, 

 a broad dorsal band between the two rows of lateral setae, a much 

 narrower ventral band between the ventral setse, a lateral band of 

 about the same width between the lateral and ventral setae on 

 either side, and finally four excessively thin strips (two dorso- 

 lateral and two ventro-lateral), one between the setigerous sacs 

 in each double row. Each band is made up of plates, perpen- 

 dicular to the circular layer, in which the fibres are arranged so 

 as to give a feather-like appearance in cross-section. Connected 

 with the longitudinal layer are delicate retractor muscles of the 

 setae, attached to the setigerous sacs where they reach the middle 

 of the circular layer. 



The septa contain muscular fibres, imbedded in much connective- 

 tissue, and taking various directions. They are covered by the 

 peritoneal epithelium of the body-cavity, with which also the longi- 

 tudinal muscle layer is lined. 



The muscle-fibres are very slender cells, united together by 

 their tapering ends. 



In locomotion the circular layer contracts so as to advance the 

 front part of the body, and the hinder part is then dragged up by 

 contraction of the longitudinal layer. The reverse movement 

 can also be effected, and complex curves described. The setae 

 act as hold-fasts, their direction and amount of protrusion being 

 regulated by their retractor and protractor muscles. 



8. The Nervous System (Fig. 20, B) consists of a ring encircling 

 the gut at the junction of the buccal pouch and pharynx, a ventral 

 cord, and numerous nerves. The dorsal side of the ring is thick - 

 2 5 



