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MANUAL OF ZOOLOGY. 



place, a chain of organically connected individuals is produced 

 all of which are nourished by the anterior portion of the primi- 

 tive worm. 



ORDER III. TUBICOLA (Cephalobranchiatd). The Annelides 

 which are included in this order inhabit tubes, which may be 

 calcareous, and secreted by the animal itself, or may be com- 

 posed of grains of sand or pieces of broken shell, cemented 

 together by a glutinous secretion from the body. The body- 

 rings are mostly provided with fasciculi of bristles set upon 

 lateral foot-tubercles or parapodia, by means of which the 

 animal is enabled to draw itself in and out of its tube. The 

 alimentary canal is loosely attached to the integument. The 

 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 Mollusca, 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 Sabella. 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. 56). 



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 



Fig. 56. Tubicola. a Serpula con- 

 tortuplicata, showing the branchiae 

 and operculum; b Spirorbis com- 

 munis. 



