JUti 



An.ei 



Fig. 74. — A, larva of Polygordius ncapolitanus 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 endodcrm radially striated, and 

 the mesoderm evenly shaded. 



C, transverse section along the plane ab in B. 



The pre-oral (/V. or. ci), post-oral (/V. or. ci), and anal {An. ci) 

 cilia, the blastocoele [Bl. ca-l), stomodac'um [St. dm), and proctodaeum 

 {Pre. 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 {t) has appeared on the prostomium near the ocelli 

 {0), and a pre-anal circlet of cilia {Pr. ait. ci) is developed. 



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

 {Msd{spl) ) layers with the ccelome {C(kI) between : the septa {Sep) are 

 formed by undivided plates of mesoderm separating the segments of the 

 coelome from one another. 



d*-d', three stages in the development of the somatic mesoderm. In 

 d' it {Msd {Som) ) consists of a single layer of cells in contact with the 

 deric epithelium (Der. Epthm) : in D*^ the cells have begun to split up 

 in a radial direction : in d' each has divided into a number of radially 

 arranged sections of muscle-plates (J/. PI) and a single cell of coelomic 

 epithelium {Cccl. Epthm).- 



(a after Fraipont. ) 



