3oo 



POLYGORDIUS 



LESS. 



may be called the trunk (Fig. 74, A). The stomach 

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

 stomium, has now grown for a considerable distance into 



Br 



B 



An c 



FIG. 74. 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. ci), post-oral (Pt. or. a'.), and anal (An. ci) 

 cilia, brain (Br}, ocelli (Oc), blastocoele (BL), mouth (Mth\ stomo- 

 daeum (St. dm}, proctodseum (Prc. dm), and anus (An) as in Fig. 72, 

 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 (NpK] 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. ) 



the trunk (B, enf), so that the proctodaeum (Prc. dm) 

 occupies only the portion in proximity to the anus. 



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 



