FIG. 74. 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 (Pi: or. ci), post-oral (Pt. or. ci}, and anal (An. ci) 

 cilia, the blastocoele (BI. cat), stomodaeum (St. dm], and proctodaeum 

 (Prc. dm] are as in Fig. 71, 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 coelome (Ccel] between : the septa (Sep) are 

 formed by undivided plates of mesoderm separating the segments of the 

 coelome from one an ^ther. 



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. Epthm). 



(A after Fraipont. ) 



