300 SYNOPTIC COLLECTION. 



enlarges the burrow as the animal moves along. Fur- 

 thermore, this burrow is lined with a mucous secretion from 

 the body which soon hardens so that the sand does not 

 come in contact with the gills while the worm is in its 

 home. 



Since Arenicola digs its own tunnel it could not have 

 the delicate gills on the segments near the mouth, but the 

 anterior region in many of these tube inhabiting worms 

 is provided with a profusion of long, slender feelers and 

 branchiae, while in the middle and posterior parts of the 

 body only a few of these organs occur. This is the case 

 with Audouinia filigera (No. 742), a handsome and grace- 

 ful worm. Extending the whole length of the body of 

 this Annelid are four rows of setae. 



Most curious of all this group of worms is Chaetop- 

 terus variopedatus Ren. (No. 743). It never leaves its 

 tube l and therefore its body has become modified in 

 various ways. The outer skin is delicate and light col- 

 ored. The segments and fleshy appendages of the middle 

 and anterior regions of the body are more specialized 

 than those of the posterior region, since it is these that 

 are modified for the purposes of catching food and for 

 holding the animal in its tube. As Chaetopterus does 

 not require locomotive paddles and setae, these organs 

 are not developed, though there are stiff hairs and plates 

 which aid the animal in moving up and down its tube. 



Terebella (=Amphitrite) coiichilega Pall., is naked and 

 free-swimming when young (No. 744 a-c). There is no 

 marked differentiation of the body in these stages, but 

 the appendages are developed in the form of clusters of 

 setae on either side. The mouth opens on the ventral 

 side (b) and in front of it is developed a very long feeler 

 (b, c). Other feelers grow out (c) until in the adult (No. 

 745, model) there are many of these delicate organs 

 besides bushy branchiae ; the latter are usually red (as 



1 Benham, Cambridge Nat. Hist., II, 1896, p. 323. 



