THE TROCHOSPHERE 



297 



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

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

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



Br 



An 



FIG. 73. 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 (PL or. '), and anal (An. ci] 

 cilia, brain (Br), ocelli (Oc), blastoccele (/?/), mouth (Mth\ stomo- 

 daeum (St. dm], proctodseum (Prc. dm\ and anus (An] as in Fig. 71, 

 the enteron (Ent) has extended some distance into the trunk. 



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

 branched head-nephridium (NpK} are shown. 



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

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

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



(A after Fraipont.) 



extremity into, the blastoccele : 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 



