TYPE ANNELIDA. 



211 



prolonged into a prostomiuni of considerable size overlying 

 the mouth ; it may be short and broad as in Echiurus, more 

 elongated and slender as in Thalassema, or deeply bifurcated 

 at the extremity as in JSonellia. 



B 



d 



FIG. 110. Bonellia viridis A, ADULT FEMALE OPENED so AS TO SHOW THE 

 PRINCIPAL ORGANS ; B, male much enlarged in proportion to the female 

 (from HEBTWIG). 



c cloaca w = muscles. 



d rudimentary intestine. s proboscis. 



g respiratory trees. s (in Fig. B) spermatozoa. 



i intestine. 'od = vas deferens. 



u single uephridium which serves also as the oviduct. 



The last-named genus is interesting as affording an exam- 

 ple of sexual dimorphism, the males being small Turbellariau- 

 like organisms which live parasitically in the anterior portion 

 of the digestive tract of the female, only coming to the exterior 

 for the purpose of copulation. 



2. Order Sipunculacea. 



The Sipunculacea, to which the term Gephyrea inermes is 

 also applied, is an order including forms which lack all traces 



