XXV METAMORPHOSIS 299 



In a word, the larva is at present, as far as the trunk is con- 

 cerned, triploblastic but accelomate. 



Development continues, and the larva assumes the form 

 shown in Fig. 74, a. The trunk has undergone a great 

 increase in length and at the same time has become divided 

 by a series of annular grooves into segments or metameres, 

 like those of the adult worm but more distinct (compare 

 Fig. 66, D, p. 269). By following the growth of the larva 

 from the preceding to the present stage, it is seen that these 

 segments are formed from before backwards, i.e., the seg- 

 ment next the peristomium is the oldest, and new ones are 

 continually being added between the last formed and the 

 extremity of the trunk, or what may now be called the anal 

 segment. By this process the larva has assumed the appear- 

 ance of a worm with an immense head and a very slender 

 trunk. 



The original larval stomach (enteron) has extended, with 

 the formation of the metameres, so as to form the greater 

 portion of the intestine : the proctodaeum {Pre. dm) is 

 confined to the anal segment. 



Two other obvious changes are the appearance of a pair 

 of small slender processes (a, /) — the rudiments of the 

 tentacles — on the apex of the prostomium, and of a circlet 

 of cilia {Fr. an. ci) round the posterior end of the trunk. 



The internal changes undergone during the assumption of 

 the present form are very striking. In every fully formed 

 metamere the mesoderm — solid, it will be remembered, 

 in the previous stage — has become divided into two layers, 

 a somatic layer (b and c, Msd (som) ) in contact with the 

 ectoderm and a splanchnic layer {Msd {spl) ) in contact 

 with the endoderm. The space between the two layers 

 {Coel} is the permanent body-cavity or coelome, which is 



