PHYLUM ANNELIDA. SEGMENTED WORMS 



185 



The body wall consists of an outer cuticle, 

 which is secreted by the cells of the epi- 

 dermis just beneath it, and several mus- 

 cular layers under the epidermis. The body 

 cavity between the body wall and the intes- 

 tine is a coelom lined with peritoneal 

 epithelium. The digestive system (Fig. 99) 

 consists of the mouth, pharynx, esophagus, 

 with an esophageal gland on either side 

 opening into it, and a straight stomach-in- 

 testine extending to the anus. 



The circulatory system comprises a dorsal 

 vessel and a ventral vessel, with branches to 

 capillaries in the body wall and intestine. 

 Almost ever}' segment, except the peristo- 

 mium and the anal segment, contains a pair 

 of nephridia. In the head is a cerebral 

 ganglion, the "brain." This is joined by a 

 pair of circumesophageal connectives to a 

 pair of subesophageal ganglia and is fol- 

 lowed by a ventral nerve cord with a pair 

 of ganglia in each segment. The sexes are 

 separate. Ova or sperms arise from the wall 

 of the coelom. A trochophore larva develops 

 from the fertilized egg. 



Polychaetes 



The principal characteristics of the classes 

 Oligochaeta and Polychaeta are exhibited by 

 the earthworm (Fig. 92) and the sandworm 

 (Fig. 99) respectively. However, many varia- 

 tions from these types occur. 



The polychaetes consist largely of free- 

 living marine annelids in which typical an- 

 nelidan characters occur. The body tends to 

 be long and wormlike, and somewhat de- 

 pressed to a cylindrical shape in cross sec- 

 tion. It consists of a prostomial or head re- 

 gion, and a trunk. Segmentation is well 

 marked both internallv and externallv. The 

 outer cuticle is usually soft and moist and is 

 dependent on a wet environment for the 

 prevention of desiccation. The digestive sys- 

 tem consists of a straight tube with an an- 



FiGURE 99. Facing page, Neanthes, the sand- 

 worm. Left, anterior end of the body with dorsal 

 wall removed. Right, some details of structure. 



teroventral mouth and a posterodorsal anus. 

 The circulatory system has a dorsal vessel 

 where the blood moves forward, and a 

 ventral one where it moves backward, to- 

 gether with transverse vessels. The nervous 

 system has a dorsal "brain" in or near the 

 prostomium, and paired ventral ganglia in 

 a laddcrlike arrangement. A giant nerve fiber 

 system is usually present, consisting of longi- 

 tudinal strands that extend parallel to the 

 ventral nerve cord and function for rapid 

 response reactions. Nephridia are segmental 

 and are present in most body segments. 

 Most polychaetes are dioecious, with the 

 two sexes resembling each other; gonads 

 may occur in many segments, and ova may 

 be produced in enormous numbers. The 

 lateral appendages or parapodia are formed 

 by outpocketings of the lateral body walls; 

 they are usually conspicuous and variously 

 provided with fleshy structures such as cirri, 

 scales, and gills. The setae occur in bundles; 

 they are formed from secretions of special- 

 ized cells, and they function in locomotion, 

 tube building, food gathering, and other im- 

 portant ways. The fertilized egg develops 

 into a trochophore. 



In detail, however, there is remarkable 

 diversity among the polychaetes so that the 

 characters named above can be regarded 

 only as generalizations. Such common 

 names for families as the following illustrate 

 the variations in shape and structure that 

 mav occur: sea mouse, scale worms, fire- 

 worms, glass worms, proboscis worms, bam- 

 boo worms, gold crowns, gooseberry worms, 

 lugworms, feather dusters, and shield worms. 

 The variable structure of polychaetes makes 

 possible adaptations to many ocean habitats. 



Most polychaetes are free-living, but many 

 are partly or wholly parasitic; most are ma- 

 rine but many others live in water varying 

 in saltiness from briny to fresh; a few are ter- 

 restrial. Metamerism may be homonymous 

 (with successive rings alike), but usually 

 there is considerable departure from this 

 structure. In Chaetopterus (Fig. 100), parts 

 of the parapodia are modified to function as 

 suction disks, as a food-ball organ, as water- 



