434 



GEPHYREA 



tance the lining of the mouth, the anus, and the two generative 

 pores. 



III. Order Echiuroidea. 



Anatomy. The most striking peculiarity of the Echiuroidea, 

 as opposed to the other two families of the Gephyrea, is the 

 presence of a solid dorsal outgrowth of a portion of the head, 

 forming the proboscis. The nature of this proboscis is something 



quite different from that 

 of the introvert of the 

 Sipunculoidea ; it would 

 appear to correspond to 

 an extension, in the mem- 

 bers of the last-named 

 Order, of that part of the 

 head which is dorsal to 

 the mouth and is covered 

 by a peculiar pigment- 

 epithelium, often in con- 

 tinuity with the brain. 

 In its outgrowth this 

 portion of the body has 

 carried with it the nerve- 

 ring and the vascular 

 ring, which both sur- 

 round the mouth. The 

 proboscis is found in all 

 the genera with the ex- 

 ception of the aberrant 

 genus Saccosoma. 



The body of the 

 female Bonellia viridis, 

 one of the best known 



species of Echiurids, is shaped like a small sausage, and is usually 

 about 2 inches long. The proboscis arises from the anterior 

 end, and is extremely extensible. At the distal end the proboscis 

 splits into two short arms, which are often recurved; along the 

 whole ventral surface runs a groove lined with cilia, which by the 

 approximation of its edges can be converted into a tube. At 

 the bottom of the proboscis the groove opens into the mouth. 



Fio. 220. A, Bonellia viridis Rol., 9 ; B, B. . 

 ginosa. Both nat. size. a. Grooved proboscis ; 

 b, mouth ; c, ventral hooks ; d, anus. 



