ANNULOSA I ANNELIDA. 2 1 3 



commonly known as Sand-worms and Sea-worms, together with 

 the familiar Sea-mice. 



The integument is soft, and the body is very distinctly 

 divided into a great number of rings or segments, each of 

 which, in the typical forms, possesses the following structure. 

 The segment consists of two arches, a lower or " ventral arc," 

 and an upper or " dorsal arc," with a " foot-tubercle " on each 

 side. Each foot-tubercle consists of an upper process, or 

 " notopodium," and a lower process, or " neuropodium," each 

 of which carries a tuft of bristles, or " setae," and a species of 

 tentacle termed the "cirrhus" (fig. 77). 



The anterior extremity of the body is usually so modified 

 as to be distinctly recognisable as the head, and is provided 

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

 and are, therefore, not comparable with the antennae 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 " chylaqueous 

 fluid " which " performs one of the functions of an internal 

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

 cutaneous muscles, the power of voluntary 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 branchiae 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 branchiae, 

 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 

 reproductive organs. 



In the Sea-mouse (Aphrodite, fig. 81, B), the back is covered 

 with a double row of membranous imbricated plates, which 



