194 MANUAL OF ZOOLOGY. 



the next order of Annelida — viz. the Errantia. It appears, 

 however, more probable that the stationary condition of the 

 adult Tubicola should rather be regarded as an instance of 

 " retrograde development." 



The most familiar of the Tubicola is the Serpula {ii%. 58, a), 

 the contorted and winding calcareous tubes of which must be 

 known to almost every one as occurring on shells or stones on 

 the sea-shore. One of the cephalic cirrhi in Serpula is much 

 developed, and carries at its extremity a conical plug, or oper- 

 culum, whereby the mouth of the tube is closed, when the 

 animal is retracted within it. The operculum of Serpula has a 

 more than ordinary interest in the fact that it is the only in- 

 stance in the Annelida in which calcareous matter is deposited 

 within the integument. In Spirorbis (fig. 58, b) the shelly tube 

 is coiled into a flat spiral, one side of which is fixed to some 

 solid object. It is of extremely common occurrence on the 

 fronds of sea-weed and on other submarine objects. 



Equally familiar with Serpula is Terebella, the animal of 

 which is included in a tube composed of sand and fragments 

 of shell, cemented together by a glutinous secretion. In the 

 Sabellida the tube is composed of granules of sand or mud. Jn 

 Pectinaria the tube is free, membranous, or papyraceous, and 

 in the form of a reversed cone of considerable length. 



Order IV. Errantia {Nereidea).- — This order comprises 

 free Annelides,* which possess setigerous foot-tubercles. The 

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

 nal branchias, 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 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. 56). 



The anterior extremity of the body is usually so modified 

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



* Fritz MuUer describes an errant Annelide belonging to the Amphinomida 

 as living parasitically within the shell of the common Barnacle (Lepas), 

 showing that the members of this group may sometimes lose their free habit. 



