PHYLUM COELHELMINTHES (ANNELIDA) 151 



terminal anus through an intestine. The region between the 

 digestive tube and the outer body wall is filled with a gelatinous 

 substance through which run strands of muscle and nerve and 

 the tubules of the protonephridial system. Near the posterior 

 extremity of the early trochophore, there usually occur a pair 

 of cells called the teloblasts which are the forerunners of the 

 mesoderm. As the larva elongates, these teloblasts continue 

 to divide, forming bands of mesoderm cells on either side of the 

 digestive tract. With the elongation of the posterior cone of the 

 trochophore in the transformation of the larva into the adult 

 worm, these mesoderm bands become divided into primitive 

 segments (Fig. 76 C) within which the coelomic cavities later 

 make their appearance. The segments of the young worm thus 

 formed are usually provided with provisional setae which are 

 later thrown off and replaced by 

 the permanent setae. 



Both free-living and sessile 

 polychaetes are found. Modifi- 

 cations of body form associated 

 with these differences in habits 

 have furnished a basis for the 

 separation of two orders. 



Free Living. — In the order 

 Errantia are included all of the 

 free-swimming polychaetes with a 

 well-developed head but with all 

 the remaining segments practi- 

 cally homonomous, bearing para- 

 podia of approximately uniform 

 character. The pharynx is ever- 



,•11 -1 e ii u ■ c Fig. 77. — Lateral view of a ser- 



tible and frequently bears a pair of ^^^.^ ^^^^ Spirorbis. removed from 



formidable jaws which are used in its shell. The ovoid bodies within 

 , • AT • iU 1 the club-shaped operculum are 



capturmg prey. Nereis, the clam- d e v e i o p i n g e m b r y o s. (After 

 worm, is one of the commonest ciaparede). 

 examples of this order. Aphro- 



dita has the dorsal surface so covered with bristles as to war- 

 rant the common name sea mouse. Lepidonotus bears twelve 

 pairs of broad overlapping scales on its back. Syllis reproduces 

 by means of lateral clusters of buds, and Autolytus undergoes 

 asexual reproduction through the formation of buds at the 

 posterior extremity. 



