METAZOA VERMES. 299 



tion, which are probably variations from the normal seg- 

 mentation. This variation of a spiral segmentation is 

 illustrated by Halla, and since it occurs in Oligochaeta 

 (see p. 305) and in Cestodes, especially in Bothriocephalus 

 latus]* it indicates a tendency toward secondary and adap- 

 tive specialization. Both the upper and lower surfaces of 

 Halla are iridescent. On either side are the parapodia 

 with bunches of projecting setae. 



Besides the species of worms already described as free- 

 swimmers there are many others which are free swimming 

 when young, but which later in life make tubes and fasten 

 them to foreign objects either temporarily or permanently. 

 Although the worms are never organically connected 

 with the tubes, yet most remain in them for the greater 

 part of the time, and in this new position develop struc- 

 tural characters fitting them for a sedentary life. 



These worms are usually grouped together under the 

 name of the Sedentaria in contrast to the Errantiaor free- 

 swimming forms. It must be borne in mind, however, 

 that there is no sharp line of demarcation between these 

 groups, since some Errantia (Eunicidae, some Polynoids) 

 make tubes and some Sedentaria are tubeless (Poly- 

 cirrus). Broadly speaking, nevertheless, the division 

 holds good, and the Sedentaria, having acquired many 

 secondary characters, are naturally placed after the 

 Errantia. 



One of these sedentary worms is Arenicola marina 

 Linn. (No. 741, model). Although not permanently 

 settled, Arenicola makes a U-shaped tunnel for itself in 

 the sand and remains in it much of the time. The para- 

 podia not being needed have disappeared, and the ante- 

 rior segments bear setae only. The central segments are 

 provided with setae and gills; the latter are protected in 

 some degree by the setae that extend beyond them, and 

 also by the swollen anterior end of the bcdy which 



1 Buchanan, loc. ctt., p. 541. 



