THE TROCHOSPHERE 



297 



Finally on either side of the lower or posterior end of the 

 stomach is a delicate tube (Fig. 73, a, Nph) opening by a small 

 aperture on to the exterior, and by a wide funnel-shaped 



ri.o 



An, ci- 



Fig.' 73. — A, living specimen of an advanced trochosphere-larva of 

 Polygordius nearjoolitanus, showing the elongation of the anal region to 

 form the trunk. 



B, diagrammatic vertical section of the same : the ectoderm is coarsely, 

 the nervous system finely, dotted, the endoderm radially striated, and 

 the mesoderm evenly shaded. 



C, transverse section through the plane ab in B. 



The pre-oral {Pr. or. ct), post-oral {Ft. or. ct), and anal {An. c£) 

 cilia, brain {Br), ocelli {Oc), blastocoele (Bf), mouth {Mth), stomo- 

 dfeum (.SV. dm), proctodaeum {Pre. dm), and anus {An) as in Fig. 71, 

 the enteron {Etit) has extended some distance into the tnmk. 



In A, slender mesodermal bands {Msd. bd) in the prostomium, and the 

 branched head-nephridium {Nph) are shown. 



In B and c the mesoderm {Msd) is seen to have obliterated the blasto- 

 ccele in the trunk-region : the ectoderm has undergone a thickening, 

 forming the ventral nerve-cord ( V. Nv. Cd). 



(a after Fraipont. ) 



extremity into the blastocoele : it has all the relations of a 

 nephridium, and is distinguished as the head-nephdridium. 



As the larva of Polygordius is so strikingly different from 

 the adult, it is obvious that development must, in this, as in 



