212 MANUAL OF ZOOLOGY. 



confined to a cincture behind the head, a transverse ventral 

 band near the tail, and a small circle round that part. The 

 head is distinguished by two red eye-specks ; new segments 

 are successively added, one behind the other, and always in 

 front of the anal one ; but as yet the embryo is apodal. The 

 tubercles and setae are next developed in the same order, and 

 a free-swimming or ' errant ' Annelida ensues. Finally, the 

 cilia of the buccal rings are lost, the young Terebella reposes, 

 and envelops itself in a mucous tube" (Owen). As the 

 young tubicolar Annelide is thus free, or " errant," before it 

 becomes finally enveloped in a tube, it is generally believed 

 that the Tubicola should be looked upon as really higher than 

 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 Serptila (fig. 79, 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. 79, 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 

 Sabellidce the tube is composed of granules of sand or mud. 

 In 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 external 

 branchiae, arranged along the back or the sides of the body. 

 They are unisexual, and the young pass through a metamor- 

 phosis. This order includes most of the animals which are 



* Fritz Miiller 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. 



