452 ZOOLOGY 



behind the prototroch and mouth, constituting what is termed 

 the ventral plate; of this plate the more superficial cells are 

 descendants of the first somatoblast one of the twelve original 

 micromeres ; and those situated more deeply are derived from 

 the second somatoblast or mesomere. A superficial thickening 

 of the ectoderm along the middle of the ventral plate is the 

 rudiment of the ventral nerve-cord (neur. pi); the deeper cells 

 divide and extend to form a pair of mesoderm bands or muscle- 

 plates, from which the muscles of the body-wall are developed ; the 

 muscular layers of the wall of the alimentary canal are derived 

 from certain of the same set of cells which migrate inwards 

 from the lower end. 



A pair of micromeres separated from the rest at an early stage 

 are destined to form the larval excretory organs, the head-kidneys or 

 larval nephridia : at first situated at the upper end, they 

 sink below the surface and migrate downwards till they come 

 to lie below the prototroch ; each then elongates, and a number 

 of vacuoles which have become formed in the interior coalesce 

 in such a way as to form a long, narrow canal. The embryo has 

 now reached the completed trochophore stage. 



The endoderm cells become arranged so as to bound a canal- 

 like space, the beginning of the lumen of the middle part of the 

 alimentary canal (oesophagus and intestine, int.), the cells subse- 

 quently giving rise to the enteric epithelium. This canal becomes 

 continuous in front with the stomodaeum, and behind with a 

 second smaller ectodermal invagination, the proctodmun, which 

 arises in the position of the former pigment-area. The part of 

 the larva behind the prototroch now elongates, and two pairs of 

 imaginations, the setiyerous sacs (set. sacs), appear at its sides: in 

 the interior of these, to which a third pair is soon added, are 

 developed setae which grow out to a great relative length as the 

 larval or provisional setce. Constrictions soon appear marking off 

 the first three segments, and at the same time the mesoderm bands 

 undergo a corresponding division into three pairs of mesoderm 

 segments. The mesoderm segments of each pair grow inwards 

 towards one another and surround the alimentary canal : in the 

 interior of each appears a cavity which is the beginning of a 

 segment or chamber of the coelome. As the two mesoderm 

 segments become closely applied to one another and unite around 

 the alimentary canal, their two cavities also come into close 

 relation, and eventually are separated from one another only by 

 thin vertical septa, forming dorsal and ventral mesenteries which 

 subsequently disappear. Successive mesoderm segments also 

 come into close relationship with one another, their cavities 

 eventually only remaining separated by thin transverse partitions, 

 which form the intersegmental septa. 



The region in front of the prototroch becomes modified to form the 



