ECHIUROIDEA. 



531 



The males and females are not distinguishable externally except 

 in Bonellia and Hamingia, in which there is a very pronounced 

 sexual dimorphism. In these genera the female has the ordinary 

 form of the species; but the male (Fig. 430) is a minute (in 

 Bonellia 1-5 mm. long) planarian-like organism which lives in 

 considerable numbers as a parasite, principally in the uterus and 

 pharynx of the female. It possesses an alimentary canal which is 

 without any opening to the exterior; the skin is ciliated, and 

 contains muscular fibres; there is a body-cavity, the lining cells of 

 which give rise to spermatozoa ; there is also a single anterior 



SP 



FIG. 431. a, Larva of Echiurus from the ventral side (after Hatschek). SP apical plate ; Prw 

 preoral ; Pow postoral ring of cilia ; KN head-kidney ; VG ventral nerve cord connected 

 with the apical plate by the long oesophageal commissures ; AS anal vesicles. I>, ventral 

 region of the Echiurus larva with segmented mesodermal bands ; SC oesophageal com- 

 missures ; Dsp dissepiments of the anterior body segments ; MS mesodermal bands ; A anus 

 (after Hatschek). 



nephridium which opens to the exterior at the front end of the 

 body, and internally into the body-cavity at its hind end. Small 

 vestiges of the anal vesicles, which open directly on the surface of 

 the body, can be made out, and in both the genera the hooked 

 setae are present. 



Development. In the Ecliiuroidea a typical trochosphere larva 

 (Fig. 431) has been observed in Echiurus and Thalassema, with a 

 strong preoral circle of cilia (Pnv), in addition to which there is also 



