296 POLYGORDIUS less. 



Before long the mouth of the gastrula closes {B), the enteron 

 {Ent) being thus converted into a shut sac. At about the same 

 time the ectoderm is tucked in or invaginated at two places 

 (C), and the two little pouches {St. dm, Pre. dm) thus formed 

 grow inwards until they meet with the closed enteron and 

 finally open into it {D), so that a complete enteric canal is 

 formed — formed, we must not fail to notice, of three distinct 

 parts: (i) an anterior ectodermal pouch, opening externally 

 by the mouth, and distinguished as the stomodceum ; (2) the 

 enteron, lined with endoderm ; and (3) a posterior ectoder- 

 mal pouch, opening externally by the anus, and called the 

 prociodceum. 



In the trochosphere (Fig. 71) the gullet is derived from 

 the stomodoeum, the stomach from the enteron, and the 

 intestine from the proctodaeum ; so that only the stomach of 

 the worm-larva corresponds with the digestive cavity of a 

 medusa : the gullet and intestine are structures not repre- 

 sented in the latter form. 



Two or three other points in the anatomy of the trocho- 

 sphere must now be referred to. 



At the apex of the dome-shaped prostomium the ecto- 

 derm is greatly thickened, forming a rounded patch of cells 

 (Figs. 71 and 73, Br), the rudiment of the brain. On the 

 surface of the same region and in close relation with the 

 brain is a pair of small patches of black pigment, the 

 eye-spots or ocelli {Oc). 



On either side of the intestine, between its epithelium and 

 the external ectoderm, is a row of cells forming a band 

 which partly blocks up the blastocoele (b and c, Msd). These 

 two bands are the rudiments of the whole of the meso- 

 dermal tissues of the adult — muscle, coelomic epithelium, 

 &c. — and are hence called mesodermal bands. 



