296 



POLYGORDIUS 



may be called the trunk (Fig. 75, A). The stomach 

 (enteron), which was formerly confined to the pro- and peri- 

 stomium, has now grown for a considerable distance into 

 the trunk (B, ent) so that the proctodseum (Prc. dm} 

 occupies only the portion in proximity to the anus. 



Br 



B 



An ci 



FIG. 75. A, living specimen of an advanced trochosphere-larva of 

 Polygordius neapolitanus, 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. '), post-oral (Pt. or. ci], and anal (An. ci) 

 cilia, brain (r), ocelli (oc), blastoccele (Bl. ccel). mouth (Mth}> stomo- 

 dseum (St. dm), proctodaeum (Prc. dm], and anus (An) as in Fig. 73, 

 A : the enteron (Ent} has extended some distance into the trunk. 



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

 branched head-kidney (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.) 



Important internal changes have also taken place. The 

 deric epithelium or external ectoderm is for the most part 

 composed, as in the preceding stage, of a single layer of 



