ANNULOSA: ANNELIDA. 185 



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 setse are next developed in the same order, and 

 a free-swimming or " errant " Annelide 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 Serpula (fig. 56, 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 r 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. 56, 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. 



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 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 



