xvii PKOTOCHOEDATA 577 



the gill region is behind the collar-cavity altogether. The stomach 

 and intestine are separated by a diaphragm perforated by a hole, 

 round which is a wisp of long cilia which beat so as to produce 

 rotatory movements. 



As metamorphosis draws on the larva decreases in size and 

 becomes more opaque. These changes are due to a diminution in 

 size of the blastocoele and to a change in shape of the ectoderm cells, 

 which become more columnar. The longitudinal band of cilia 

 becomes indistinct, except along its posterior border; the circular 

 band remains active, but it becomes shifted farther back by a growth 



ff-P 



proa 



^^r- . M AHK _ U __^^^B^H^H 



-coe 3 



ttr 



a 



FIG. 421. Full-grown New England Tomaria seen from the left side. (After Morgan.) 



a, anus ; a.d.a, anterior dorsal ami of. the longitudinal ciliated band ; ap, apical plate ; ap.s, apical 

 string; eil.lmg, longitudinal ciliated band; coel, anterior (proboscis) coelom; coe2, middle (collar) 

 coelom ; coe 3 , posterior (trunk) coelom ; g.p, gill pouches ; int, intestine ; o, mouth ; oes, oesophagus ; 

 per, pericardial sac ; p.l.a, posterior lateral arm of the longitudinal ciliated band ; pr.o.a, prae-oral arm 

 of the longitudinal ciliated band ; st, stomach ; t.tr, telotroch ; w.p, water-pore. 



of the region of the body intervening between it and the longi- 

 tudinal band. 



The larva now drops to the bottom and glides over it by the 

 help of the cilia of the circular band. The anterior region of the 

 body grows in length and becomes conical, and gradually takes on 

 the shape of the proboscis of the adult. The posterior wall of the 

 anterior coelom or proboscis-cavity becomes curved forwards and 

 thrown into a number of folds, and this constitutes the excretory 

 tissue or head kidney of the adult (ex, Fig. 422, A and B). The 

 posterior border of the longitudinal band becomes slightly invaginated 

 and marks the front edge of the collar region ; the hinder limit of 

 this region is marked by a new transverse groove, which now appears 



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