Msd (fern) 



v.m 



fr.an.ci- 



An.ci 



FIG. 76. A, larva of Polygordius neapolitanus in a condition inter- 

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

 having 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 (PL or. cz), and anal (An. ci) 

 cilia, the blastoccele (Bl. cosl}, stomodaeum (St. dm], and proctodaeum 

 (Prc. dm] are as in Fig. 73, A and B : the enteron now extends through- 

 out the segmented region of the trunk. 



A pair of tentacles (t] has appeared on the prostomium near the ocelli 

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



The mesoderm has divided into somatic (J\hd (som) ) and splanchnic 

 (Msd (spl) ) layers with the ccelome (Ccet) between : the septa (sep) are 

 formed by undivided plates of mesoderm separating the segments of the 

 ccelome from one another. 



D 1 -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. Epthm} : 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 ccelome 

 epithelium (Ccel. Epthm}. 



(A after Fraipont.) 



