THE CLASSIFICATION OF ANIMALS 43 



from the cells of the skin which lie outside the longitudinal 

 muscles. One purpose of the coelom is to allow the gut and the 

 skin to be moved independently of one another ; whereas in 

 the " worms " which we have heretofore considered, the whole 

 body moves in one piece, so to speak, an Annelid might 

 almost be considered as an animal within an animal, for the 

 motions of the gut go on independently of those of the outer 

 skin. The muscles of the gut undergo contraction one after 

 the other in such a way as to cause a wave of contraction to 

 pass along the tube. This wave motion is called peristalsis, 

 and its effect is to push the food along the tube. Similar 

 peristalsis, it is true, takes place in the outer circular muscles, 

 and its effect is to push the watery fluid contained in the 

 coelom along from segment to segment, since the ccelomic sacs 

 communicate with each other by means of small holes beneath 

 the gut. This "outer peristalsis" lengthens the worm, and 

 enables it to push its front end onwards. When progress 

 has thus been effected, the front end is fixed in the earth by 

 its bristles, and the body is then drawn up by the contraction 

 of the longitudinal muscles ; the hind end is then fixed in the 

 ground, 4 and by a repetition of the outer peristalsis the advance 

 of the front end is repeated ; so the worm moves. It must be 

 borne in mind that a primary body-cavity or series of blood- 

 spaces exists in Annelida as well as the secondary body-cavity. 

 This primary body-cavity takes the form of a series of branched 

 tubes which contain a red fluid, and which are distributed to 

 all the organs of the body. This red fluid is, of course, the 

 " blood " of the worm. Nearly all worms of the phylum 

 Annelida are burrowers at some stage of their existence; 

 the majority burrow in the sand and mud at the bottom of the 

 sea, and only rise to the surface when they have ripened their 

 reproductive cells and are about to discharge them. At such 

 times they often appear in countless numbers, and furnish an 

 important source of food-supply to the natives of the Pacific 

 islands, who call them " Palolo." These reproductive cells are 

 produced from the walls of the ccelomic sacs, and in some few 

 cases are only discharged by the breaking of the body ; but in 

 most cases definite openings, termed reproductive pores or 

 coalomiducts, exist, which allow of the discharge of the re- 

 productive cells. No annelid stands convicted of causing 



