THE POLYCH^ETA. 229 



genera. The bristles are either formed of one continuous piece or are jointed. They may be 

 hair-like, setaceous, or slender, and tapering insensibly to the end, lanceolate or swollen. The 

 branchue in the cephalous Worms are attached to the base of the foot, on the upper or dorsal 

 side, and are either restricted to a certain number of segments, or they are found on all. They 

 are either arborescent, combed on one side, flat, and veined, or they may be filamentary. At the base of 

 the branchiae, or in portions of the lateral trunks, are "hearts," the direction of the fluid 

 being from behind forwards in the dorsal vessel, and the opposite in the ventral trunk. There 

 are numerous branches to the trunks in most, but not in all, the Polychseta. No segmental 

 organs excretory have been discovered in the majority of these Worms, but they do exist in 

 some, as short ciliated canals opening on the parapodia or ventral surface, or as cavities with 

 glandular walls. They may excrete a renal deposit, or may have to do with reproduction. The 

 nervous system consists of a chain of ganglia, one pair for each segment, connected together by 

 longitudinal and transverse bands which diverge below the cerebral ganglions and the succeeding pair, 

 to allow of the passage of the oesophagus. The commissural bands differ in length in the many genera, 

 and some fusion of the ganglia also occurs. An extensive series of nerves is given off to the 

 viscera. 



The general cavity of the body, the perivisceral, contains a fluid and colourless corpuscles, 

 except in two genera ; and this fluid is continuous with that of the parapodia, and their accessory 

 structures, they being more or less hollow, and in relation to the perivisceral cavity. Cilia, and 

 the movements of the body, produce the circulation of this fluid. Branchiae are represented by 

 ciliated spots on the dorsal side of the bases of the parapodia, or ciliated tubercles may arise from 

 the spot, and it is within them that the caeca of the alimentary canal terminate. There may be 

 filiform branchiae or there may be branchial tufts. The pseudo-haemal system may or may not be 

 present, and when it is found, as, for instance, in the genera where tufted branchiae exist, loops 

 of the great vascular trunks enter them. These trunks are dorsal and ventral, connected by 

 transverse branches, and may be rhythmically contractile. They are large, squamous, lobe-like, 

 or tubercular. In many acephalous genera the branchiae are placed in front, in tufts. 



The cirri are simple, soft, tapered filaments, or papillary processes attached to the dorsal 

 and ventral lobes, at or near the base. Their office appears to be tactile, and they may be considered 

 as the tentacles of the body. 



In the Polychaeta the foot and its accessory structures are well developed on either side of 

 certain segments. The group, as a rule, are cephalous, and their alimentary canal is almost always of 

 the same length as the body, and extends without marked distinction into stomach, and con- 

 voluted intestine, from the mouth to the anus. In some genera, long caeca are given off from 

 each side of the alimentary canal, and are sometimes much convoluted. The pharynx is muscular, 

 and when turned out as a proboscis is in some instances as long as the body. There are papillae 

 on it, and, in some cases, horny teeth, which are carried and implanted in the muscular 

 tissue. Eyes and auditory vesicles exist; the former are simple expansions of nerve embedded in 

 pigment, and are usually on the prestomial segment ; but in some genera they are on the segments and 

 tentacles. Some species have them on the tail end, and the locomotion is then with the posterior 

 part forwards. Otoliths have not been satisfactorily made out in the Errantia, but they have 

 been discovered in the Lob Worm. 



SUB-ORDER ERRANTIA. 



The majority of these many-bristled Worms lead an active, predatory life ; have a distinct head, 

 carrying eyes, tentacles, and usually tentacular cirri. The body is not divided into different regions, 

 and the highly-developed parapodia are used as oars. The gullet is probosciform and armed, and 

 when the branchiae exist, they are tufts or comb-shaped projections on the dorsal lobes of the feet. 

 They swim freely, and only a few inhabit temporarily very thin membranous tubes. In some genera 

 there are flap-shaped processes to the body, which are called elytra and flap-shaped branchiae. 



The Errantia are very numerous in individuals. There is a host of genera, and no less than 

 twelve families, some of which are again subdivided. 



