POLYGORDIUS 



the formation of an enteron or digestive cavity the polyplast 

 becomes a gastrula (see p. 260) which by further develop- 

 ment is converted into a curious, free-swimming creature 

 shown in Fig. 73, A, and called a trochosphere. 



oo 



A 



and 



Pt-.or.CL 

 dm 



FIG. 73. A, larva of Polygordius neapolitanus in the trochosphere 

 stage : from a living specimen. 



B, diagrammatic vertical section of the same : the ectoderm is dotted, 

 the endoderm radially striated, the mesoderm evenly shaded, and the 

 nervous system finely dotted. 



C, transverse section through the plane ab in B. 



The body-wall consists of a single layer of ectoderm cells, which, at 

 the apex of the prostomium (upper hemisphere) are modified to form the 

 brain (Br) and a pair of ocelli (oc). 



The enteric canal consists of three parts : the stomodaeum (st. dm], 

 opening externally by the mouth (Mth), and lined by ectoderm ; the 

 enteron (Ent) lined by endoderm ; and the proctodseum (Prc. dm], 

 opening by the anus (An) and lined by ectoderm. 



Between the body-wall and the enteric canal is the larval body-cavity 

 or blastoccele (BL cccl). 



The mesoderm is confined to two narrow bands of cells (B and C, 

 Msd) in the blastoccele, one on either side of the proctodaeum ; slender 

 mesodermal bands (Msd' ) are also seen in the prostomium in A. 



The cilia consists of a prse-oral circlet (Pr. or. ci) above the mouth, a 

 post-oral circlet (PL or. ci) below the mouth, and an anal circlet (An. 

 ci} around the anus. 



(A after Fraipont.) 



The trochosphere, or newly-hatched larva of Polygordius 

 (Fig. 73, A) is about \ mm. in diameter, and has something 

 the form of a top, consisting of a dome-like upper portion, 

 the prostomium, produced into a projecting horizontal rim ; 



