Pr.or.cl 



Msd (/Sfetnf\ 



Y.TSf 

 Pr.an.cl 



An.ci 



FIG. 75. A, larva of Polygordius neapolitanus in a condition inter- 

 mediate between the trochosphere and the adult worm, the trunk-region 

 being elongated and divided into metameres. 



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 along the plane ab in B. 



The pre-oral (Pr. or. ci), post-oral (Pi. or. ci), and anal (An. ci) 

 cilia, the blastocoele (Bl. ccel), stomodaeum (St. dm), and proctodceum 

 (Prc. dm) are as in Fig. 72,. A and B : the enteron now extends through- 

 out the segmented region of the trunk. 



A pair of tentacles (/) has appeared on the prostomium near the ocelli 

 (o), and a pre-anal circlet of cilia (Pr. an. ci) is developed. 



The mesoderm has divided into somatic (Msd (som) ) and splanchnic 

 (Msd (spl) ) layers with the ccelome (Ccel) between : the septa (Sep) are 

 formed by undivided plates of mesoderm separating the segments of the 

 ccelome from one another. 



D^D 3 , three stages in the development of the somatic mesoderm. In 

 D 1 it (Msd (Som) ) consists of a single layer of cells in contact with the 

 deric epithelium (Der. Epthni) : in D 2 the cells have begun to split up 

 in a radial direction : in D 3 each has divided into a number of radially 

 arranged sections of muscle-plates ( M. PI) and a single cell of ccelomic 

 epithelium (Ccel. Epthni), 



(A after Fraipont. ) 



