xxvi METAMORPHOSIS 299 



In the trochosphere (Fig. 72) the gullet is derived from 

 the stomodceum, 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. 72 and 74, Br\ the rudiment of the brain. On the 

 surface of the same region and in close relation wi^h 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 blastoccele (B and c, Msd\ These 

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

 dermal tissues of the adult muscle, ccelomic epithelium, 

 &c. and hence called mesodermal bands. 



Finally on either side of the lower or posterior end of the 

 stomach is a delicate tube (Fig. 74, A, NpJi] opening by a 

 small aperture on to the exterior, and by a wide funnel- 

 shaped extremity into the blastoccele : it has all the relations 

 of a nephridium, and is distinguished as the head-kidney. 



As the larva of Polygordius is so strikingly different from 

 the adult, it is obvious that development must, in this, as in 

 several cases which have come under our notice, be accom- 

 panied by a metamorphosis. 



The first obvious change is the elongation of the conical 

 anal region of the trochosphere into a tail-like portion which 



