ANNULOSA : ANNELIDA. I95 



with eyes, and with two or more feelers, which are not jointed, 

 and are, therefore, not comparable with the antennse of Crus- 

 tacea and Insects. The mouth is placed on the inferior sur- 

 face of the head, and is often furnished with one or more pairs 

 of horny jaws, working laterally. The pharynx is muscular, 

 and forms a sort of proboscis, being provided with special 

 muscles, by means of which it can be everted and again re- 

 tracted. In most there is no distinction between stomach and 

 intestine, and the epithelium of the alimentary canal, like that 

 of the preceding orders, is ciliated» The perivisceral cavity 

 is filled with a colourless corpusculated fluid — the " chylaque- 

 ou£ fluid" — ^which " performs one of the functions of an inter- 

 nal skeleton, acting as the fulcrum or base of resistance to the 

 cutaneous muscles, the power of volurjtary motion being lost 

 when the fluid is let out." — (Owen.) 



The pseudo-haemal system is well developed, and consists 

 essentially of a long dorsal vessel, and a similar ventral one, 

 connected by transverse branches, and furnished at the bases 

 of the branchias with pulsating dilatations. The contained 

 fluid is mostly red, but is yellow in Aphrodite and Polynoe. 



Respiration is carried on by means of a series of external 

 branchiae or gills, arranged in tufts upon the sides of the body 

 on its dorsal aspect, along the middle of the body only, or 

 along its entire length. From the position of the branchise, 

 the members of this order are often spoken of as the " Dorsi- 

 branchiate " (or more properly " Notobranchiate ") Annelides. 

 The " segmental organs," with few exceptions, communicate 

 with the perivisceral cavity internally, and in certain segments 

 they are always specialised to act as efferent ducts for the re- 

 productive organs. 



In the Sea-mouse { Aphrodite, iig. 60, B), the back is covered 

 with a double row of membranous imbricated plates, which 

 are called " elytra," or " squamse," and respiration is eff'ected 

 by the periodical elevation and depression of these plates, 

 whereby water is alternately admitted into, and expelled from, 

 a space beneath them. This space is separated by a membrane 

 from the perivisceral cavity below, and contains the gills in the 

 form of small fleshy crests. The pharynx is thick and muscular, 

 and can be everted "ike a proboscis, and the intestine has a 

 number of lateral branched cseca. 



The nervous system in the Errantia has its typical form, 

 consisting of a double gangliated ventral cord, two ganglia of 

 which are appropriated to each segment. The pras-oesopha- 

 geal, or cerebral, ganglia are of large size, and send filaments 

 to the ocelli and feelers. 

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