ANNULOSA: ANNELIDA. 267 



comprises /V<^ Annelides,* which possess setigerous foot-tubercles. 

 The respiratory organs are generally in the form of tufts of ex- 

 ternal branchice, arranged along the back or the sides of the body. 

 The sexes are distinct, and the young pass through a metamor- 

 phosis. This order includes most of the animals which are 

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

 with the familiar Sea-mice. 



The integument is soft, and the body is distinctly divided 

 into a 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 "noto- 

 podium," and a lower process, or " neuropodium," each of 

 which carries a tuft of bristles, or "setae," (rarely, a single 

 bristle) and a species of tentacle termed the "cirrhus" (fig. 

 126). 



The outer, cuticular layer of the body is generally more or 

 less chitinous, and is often iridescent. Below this is a mus- 

 cular layer, by which the movements of the animal are effected, 

 and which encloses the " perivisceral cavity." This cavity 

 runs the whole length of the body, and is lined by a special, 

 often ciliated membrane, which is reflected upon the alimen- 

 tary canal and other internal organs. It is usually more or 

 less subdivided by imperfect partitions, and is filled with an 

 albuminous fluid containing floating corpuscles, and corre- 

 sponding with the blood. This so-called " chylaqueous fluid," 

 " 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 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 



* Fritz Miiller describes an errant Annelide belonging to the Amphino- 

 mid<z as living parasitically within the shell of the. common Barnacle 

 (Lepas], showing that the members of this group may sometimes lose their 

 free habit. 



