METAMORPHOSIS. 



69 



Argiqpe, these latter are wanting. The two shell-valves now soon 

 form on the outer surface of the mantle-lobes as cuticular secretions. 

 The pedal region of the larva becomes the peduncle of the adult, 

 and two large groups of muscles which can be recognised even in the 

 larva (Fig. 32 B) become changed into the ventral peduncular 

 muscles. In Liothyrina and Terehratulina there is also a pair of 

 dorsal peduncular muscles in the larva. The two pairs of muscles 

 which end at the bundles of setae become the shell-adductors. The 

 pair of muscles which, in Fig. 32 B, lies on either side of the posterior 

 end of the digestive tract, represents the rudiment of the divaricators. 

 This pair divides later into a pair of dorsal and a pair of ventral 

 divaricators. 



Fig. 31.— Stages in the attachment and metamorphosis of the larva of Terehratulina (after 

 Morse), b, tufts of provisional setae ; c, cephalic region ; th, thoracic region ; p, peduncular 

 region of the body. 



The metamorphosis of the cephalic region of the larva is the 

 most obscure. In comparing this larva with that of Phoronis, we 

 vshould expect that this section would give rise merely to the integu- 

 mental fold above the mouth (epistome) and to the supra-oesophageal 

 ganglion (see diagram, Fig. 34). According to Kowalevsky, however, 

 it appears, on the contrary, that the rudiment of the oesophagus 

 (Fig. 33 A, oe) develops as an ectodermal invagination in the region 

 of the cephalic lobe, and the latter, with its eye-spots, is for some 

 time longer recognisable within the attached larva (Fig. 33 B). If 

 this is the case, we should have to attribute to the ciliated ring, 

 which runs round the cephalic lobe of the larva, a postoral position. 



