528 



ANNELIDA, 



deemed advisable, for reasons set forth on another page (p. 533), 

 to break up this class, and to establish the Sipunculoidea as an 

 independent phylum of the animal kingdom, with affinities to 

 Phoi'onis and perhaps to the Annelida, and the Ecliiuroidea as an 

 aberrant class of the phylum Annelida. 



The Ecliiuroidea have a somewhat cylindrical body, which is 

 prolonged anteriorly into a long, highly contractile preoral proboscis 

 (Figs. 428, 429). They are exclusively marine, and for the most 

 part live in holes and fissures in f ocks and between stones ; Echiurus, 

 however, frequents sand or mud, in which it forms tubes with two 



openings. Many members of 

 the class appear to have the 

 habit of frequently changing 

 their residence. The proboscis 

 has a ciliated groove on its 

 ventral surface, which leads 

 behind into the mouth situated 

 at its base. The animals appear 

 to subsist on organisms and 

 organic particles, which are 

 brought to the mouth by the 

 currents of water driven along 

 the groove of the proboscis by 

 the cilia. The anus is posterior 

 and terminal. 



There are almost always two 

 hooked setae on the ventral 

 surface not far from the front 

 end (Fig. 428 H) ; they are 

 embedded in pits of the skin, 

 by the lining of Avhich they 



are secreted in the typical annelidan manner. In Echiurus there 

 is in addition a single or double row of setae round the hind end; 

 these are the anal setae. A short distance behind the hooked setae 

 are the openings of the anterior nephridia ; these vary in number 

 from four pairs to a single one. 



The skin is covered with small papillae, which are often arranged 

 in rings. The body-wall is highly muscular, and consists of the 

 usual layers. 



The central nervous system consists, as in the Sipunculoidea, 

 of a single ventral cord, lying entirely within the body-wall. In 



FIG. 428. Young Echiurus, ventral view (after 

 Hatschek). mouth at the base of the 

 proboscis; SC circumoesophageal commis- 

 sures ; BS ventral cord ; A anus ; H ventral 

 hooks. 



