ANNULOSA : ANNELIDA. 165 



ral tufts, arranged in a funnel-shaped or spiral form. Each 

 filament is fringed with vibrating cilia, and the tufts are richly 

 supplied with fluid from the pseudo-ha3mal system. There is 

 no special apparatus required to drive the blood back to the 

 heart, but this is effected by the contractile power of the gills 

 themselves. From the position of the branchiae upon, or near, 

 the head, the Tubicola are often known as the ' cephalo- 

 branchiate ' Annelides (fig. 46). 



Reproduction in the Tubicola is generally sexual, the sexes 

 being in different individuals ; but spontaneous fission has also 

 been observed. As regards their development, the process has 

 been thus described, as it occurs in Terebella : The embryo, 

 which is at first a free- swimming, ciliated body, ' lengthens, 

 and the cilia, which were at first generally diffused, become 

 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 setaB 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 re- 

 poses, 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, how- 

 ever, more probable that the stationary condition of the adult 

 Tubicola should rather be regarded as an instance of ' retro- 

 grade development.' 



The most familiar of the Tubicola is the Serpula, the con- 

 torted and winding calcareous tubes of which must be known 

 to almost every one, as oc- 

 curring on shells or stones 

 on the sea- shore. One of the 

 cephalic cirrhi in Serpula is 

 much developed, and car- 

 ries at its extremity a coni- 

 cal plug, or operculum, 

 whereby the mouth of the 

 tube is closed, when the 

 animal is retracted within it. Fig> 46> _ serpnia. 



Equally familiar with Ser- 

 pula is Terebella, the animal of which is included in a tube 

 composed of sand and fragments of shell, cemented together 

 by a glutinous secretion. 



