o78 CKESSWKLl, SHKAKKi;. 



the proctodasum. Only in the troclioplioral stag-e does the 

 coeloinesoblast divide, giving rise to the mesoblast bands, 

 wliich gradually grow np the head-kiduey ducts to the region 

 of the oesophagus. There is relatively a considerable period 

 during the troclioplioral stage, when the larva is without 

 mesoderm bands, and the rudiments of the bands are repre- 

 sented by a single cell on either side of the gut-wall in the 

 anal region. 



7. The Early Teochophokal Stages. 



In part tlie earlj^ troclioplioral stages have been considered 

 in the foregoing section. Before the completion of gastrula- 

 tion the larva begins to assume the shape of the trochophore. 

 Figs. 9, 10, 12, 13, 15 and 16 show tlie shape of the. early 

 larvfB ; of these probably fig. 12 is the most typical. In these 

 figures the upper and lower larval hemisphere is dome-shaped 

 and rounded, as compared with the pointed and more conical 

 appearance of the mature larvto shown in figs. 1, 2, and o. 

 The apical cilia, cilia of the mouth, prototroch and paratroch, 

 are, for the sake of simplicity, not shown in these figures, 

 which are drawn from fixed material, and are therefore more 

 granular looking than the living larvti}. These stages are 

 derived from the gastrula about the twentieth to the thirtieth 

 hour of development. At this time there is a great thinning 

 out of the tissues, and. the larva rapidly increases in size. In 

 the region of the prototroch a very active prolifei-ation of the 

 cells is taking place, by which the gastrula is lengthened out 

 into the conical dome-shaped larva. The ])riniitive archen- 

 teron becomes sharply divided into the cylindrical oesophagus, 

 cubical stomach, and narrow gut. The cells of its walls are 

 seen dividing rapidly. The inner surface of the oesophagus 

 secretes a cuticle, as in Thalassema (Text-fig. 19). The 

 archenteron is lined throughout with strong cilia. Those of 

 the oesophagus are remarkably long and powerful. The inner 

 wall of the stomach is covered uniforndy with fine cilia, which 

 keep the food contents in constant motion. The cilia of the 



