TUBICOLA; TEREBELLA, AMPHITRITE, PECTINARIA. 303 



animals of this group, which do not form their tubes by a calca- 

 reous exudation from their own bodies, but by cementing together 

 particles of shell, sand, small pebbles, &c., by means of a gluti- 

 nous secretion. We may first notice the Sabella, which seems 

 to have some power of exuding calcareous matter ; and forms its 

 shell partly of this material, and partly of granules of clay or 

 fine mud. In the arrangement of its gill-tufts, it bears a strong 



resemblance to the Serpula. 

 The Terebella forms its tube 

 entirely by the agglutina- 

 tion of grains of sand, pieces 

 of shell, &c. ; and some 

 species live in groups, so 



Fio. 630. TKRKBBLLA MEDUSA, in its tube. that their clustering-tOge- 



ther forms solid masses, 



which may go on increasing to a considerable size. The gills are 

 here smaller, and are placed behind the head, having the form of 

 small arborescent tufts ; and the head is furnished with numerous 

 tentacula, which are capable of being greatly extended. These 

 probably assist the gills in the aeration of the blood, as they pre- 

 sent a large surface to the water ; but their chief use appears to 

 be the prehension of food. The shelly tubes usually have little 

 branches near their summit, through which the gills and the ten- 

 tacula pass out. The Tubicola of the genus Amphitrite are 

 distinguished by their large 

 golden-coloured setae, disposed 

 in a comb-like series, or in a 

 crown, or in one or several 

 ranges on the front of the head ; 

 these probably serve them for 

 defence, and may give assistance 

 in locomotion. Many of these 

 animals, belonging to the sub- 

 genus Pectinaria, are less con- 

 fined to one spot than the pre- 



j.- ,1 ,i_ i i 



ceding; their tubes, which are 



composed of fine grains of sand cemented together with great 



regularity, are simple cones open at both endfc and not attached ; 



FIG 631 TKREBELLA VARIABILIS. 



