ANNULOSA : ANNELIDA. 



195 



Tubicola are unisexual, and the young pass through a meta- 

 morphosis. 



When the tube of a Tubicolar Annelide is a true calcareous 

 secretion from the body of the animal, it is, nevertheless, 

 readily distinguished from the shell of the MoUusca, by the- fact 

 that there is no organic connection of any kind between the 

 animal and its tube. 



The pseudo-haemal system has its usual arrangement, and 

 the contained fluid is usually red in colour, but is olive-green 

 in Sabdla. The respiratory or- 

 gans are in the form of filament- 

 ous branchiae, attached to, or 

 near, the head, generally in two 

 lateral tufts, arranged in a funnel- 

 shaped or spiral form. Each fila- 

 ment is fringed with vibrating 

 cilia, and the tufts are richly sup- 

 plied with fluid from the pseudo- 

 haemal system. There is no spe- 

 cial apparatus required to drive 

 the blood back to the heart, 

 but this is effected by the con- 

 tractile power of the gills them- 

 selves. From the position of 

 the branchiae upon, or near, the 

 head, the Tubicola are often 

 'known as the " cephalobranchiate " Annelides (fig. 58). 



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 Tercbella : — 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 va. 

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

 tubercles and setas 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 beheved 

 that the Tubicola should be looked upon as really higher than 



Fig. 58. — Tubicola. a Serpula con- 

 iortupiicata, showing the branchiae 

 and operculum ; b Spirotbis com- 

 fnunis. 



