166 MANUAL OF ZOOLOGY. 



ORDER IV. ERRANTIA (Nereidea). This order comprises 

 free Aniielides, which possess setigerous foot-tubercles. The 

 respiratory organs are generally in the form of tnfts of exter- 

 nal branchiae, arranged along the back or the sides of the 

 body. They are unisexual, and the young pass through a 

 metamorphosis. 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 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 aii upper process, or 

 * notopodium,' and a lower process, or ' neuropodium,' each of 

 which carries a tuft of bristles, or ' seta?,' and a species of ten- 

 tacle termed the ' cirrhus ' (fig. 44). 



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 antenna 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, w r orking 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 ' chyl- 

 aqueous 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-hasmal 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 

 branchia? 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. 



In the Sea-mouse (Aphrodite), the back is covered with a 



