THE TROCHOSPHERE 



293 



embryo is gradually converted into a curious free-swimming 

 creature shown in Fig. 7 1 , a, and called a trochosphere. 



The trochosphere, or newly-hatched larva of Polygordius 

 (Fig. 71, a) is about \ mm. in diameter, and has something 



Fig. 71. — A, larva of Polygorditts 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 (^r) and a pair of ocelli {,oc). 



The enteric canal consists of three parts : the stomodaeum {St. dm), 

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

 enteron (Eut) lined by endoderm ; and the proctodEEum (Pre. dm), 

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



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

 or blastocoele (Bl. cal). 



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

 Alsd) in the blastocoele, one on either side of the proctoda^um ; slender 

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



The cilia consist of a prae-oral circlet (Fr. or. ci) above the mouth, a 

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

 ci) around the anus. 



(a after Fraipont. ) 



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

 the prostomium, produced into a projecting horizontal rim ; 

 of an intermediate portion or peristomium, having the form 

 of an inverted hemisphere ; and of a lower somewhat conical 



