METAZOA VERMES. 301 



represented by the model), owing to the blood contained 

 in them. The tube (No. 745) of the adult Terebella is 

 sometimes fifteen inches long and is usually made of 

 fragments of shells, sand grains, etc., cemented together 

 by a secretion of the body. These substances the worm 

 prefers, and it shows discrimination in selecting materials, 

 refusing certain substances altogether, as pointed out by 

 Dalyell. 1 From the larger end the many extremely long 

 feelers and the feathery, brilliantly colored gills are 

 extended, as already stated ; the latter are borne on only 

 a few of the anterior segments. One of the tentacles 

 has become differentiated into an operculum or "stopper" 

 which closes the tube in time of danger. 



The bristles are found on the forward segments only,, 

 having disappeared from the posterior region of the body 

 where they would be of no use to a tube-inhabiting animal. 



Stylarioides monilifer (No. 746), is an instructive form. 

 The plump, rounded, anterior region of the body has 

 nearly lost the boundary lines of the segments, though 

 these are distinct on the tapering posterior region. The 

 head is extremely differentiated. The tentacles are sur- 

 rounded by long bristles which reflect the most brilliant 

 prismatic hues. Many of the setae on the anterior region 

 of the body exist as vestiges like the lines of demarcation 

 between the segments, while the setae on the posterior 

 region are more regular and bushy. 



Cistenides, or as it was formerly called, Pectinaria (No. 

 747), has a symmetrical tube reminding one in shape 

 of the shell of Dentalium (see No. 534). It is made 

 of tiny pebbles which are fitted together with extreme 

 neatness. There is an opening at the smaller posterior 

 extremity, and the head extends from the larger anterior 

 end ; this is provided above with prismatic bristles. 

 Though placed with the Sedentaria, Cistenides is not a 

 sedentary worm, since it travels about freely, carrying 

 its tube with the smaller end pointing upward. 



1 Ann. and Mag. Nat. Hist., (6), XIII, 1894, p. n. 



