168 GEPHYREA [CH. 



the body. (3) The Priapuloidea which agree with the Sipunculoidea 

 in having no prostomium and in having the front region of the 

 body introversible, but in which the anus is terminal. 



Now the development of Echiuroidea demonstrates that this 

 group really does belong to the Annelida, for the larva is distinctly 

 divided into segments and each segment contains a section of the 

 coelom. Since they possess at least a pair of chaetae they are 

 Chaetopoda, and since the generative cells are developed from the 

 wall of the coelom covering the ventral blood-vessel along the whole 

 length of the body, and since the same organs act as excretory 

 organs and genital ducts, they are to be regarded as modified 

 Polychaeta. They are in fact Polychaeta which have ceased to 

 burrow and which pass their lives in one spot, feeding themselves 

 by what is brought to them by the current of water produced by 

 the cilia lining the under surface of the prostomium. In the genus 

 Bonellia, which is found in the Mediterranean, the prostomium 

 is produced at its anterior end into two muscular flaps which are 

 used for seizing prey. The prostomium of Bonellia when fully 

 extended is 10 to 15 times as long as the animal, and this is a 

 remarkable instance of the way in which any organ of the body can 

 be increased in size indefinitely by the action of natural selection if 

 it becomes of importance in obtaining food for its possessor, for it 

 must be remembered that the enormous " proboscis" of Bonellia is 

 strictly comparable with the insignificant lobe which overhangs the 

 mouth in the earthworm. 



One striking peculiarity of the Echiuroidea may be mentioned 

 and that is, that in addition to possessing anterior nephridia which 

 serve as genital ducts, they likewise possess a pair of peculiar 

 nephridia which open into the hindermost part of the alimentary 

 canal. These terminate internally in a multitude of small ciliated 

 funnels opening into the body-cavity These funnels are scattered 

 all over the surface of the nephridia. 



The Sipunculoidea are not closely connected with the Annelida. 

 The features which they have in common with them are distinctive, 

 not merely of Annelida but of a wide range of animals which retain 

 the coelom in a more or less primitive condition, and these only 

 show that both Annelida and Sipunculoidea belong to the Coelom ate 

 division of Metazoa. Since the line joining mouth and anus is to 

 be looked upon as the dorsal surface, it follows that the main part of 

 the length of these animals is to be regarded as being made up of 

 a great protrusion of the ventral surface of the body. The same is 



