ARENICOLA. 189 
series enable the animal to lengthen or shorten its body, like a concer- 
tina. But this by itself would not effect movement of the whole body. 
Further provision is usually found in hooks, suckers, or setae, which 
catch the ground or surrounding medium and prevent movement in ove 
direction, usually backwards. By this means all the expansion or 
lengthening and shortening or contraction must take place forwards and 
progress is therefore rapid. We may emphasise in this arrangement the 
~ motor function performed by the ccelom and its fluid, which may be 
conipared with the more specialised condition in the proboscis of 
Balanoglossus and the ambulacral system of Aséerias. 
The excretory organs consist of six pairs of nephridia (in 
segments 4 to 9 inclusive). These are wide tubes which 
open into the ccelom by large funnels or 
nephrostomes and to the exterior by small ne- 
phridiopores. A patch of coelomic epithelium covering each 
Excretory. 
Fig. 115.—A NEPHRIDIUM OF ARENICOLA. 
(After ASHworTH and GamBLE.) 
Gonadial 
Filaments. Excretory 
Portion. 
Nephridiopore. 
4 
> 
Vesicle or Bladder. 
of the nephridia, except the first, gives rise to the sexual 
elements, which lie free in the nutritive 
coelomic fluid and pass to the exterior by the 
nephridia. Avenzcola is dicecious and breeds in spring 
and summer. It develops by a free larval form allied to 
the trochophore. 
Reproductive. 
[ TABLE, 
