250 THE INVERTEBRATA 



closes, but in the neighbourhood of which the anus of the trocho- 

 sphere arises later. The blastopore therefore represents the ventral 

 surface of the larva. At the same time the macromeres withdraw into 

 the interior to form a second cavity, the archenteron, bringing with 

 them the cells of the fourth and fifth quartettes ( a'^, b^, c^ ; a^, b^, c^, d^). 

 The somatoblast (d^) breaks up into a large number of cells to form 

 the ventral plate . 



The change from gastrula to trochosphere (Fig. i86) follows quickly 

 and with little further cell division. The first quartette of micromeres 

 have by this time been differentiated (Fig. 185, 5) into (i) the apical 

 rosette, consisting at first of four small cells and becoming the apical 

 organ of the trochosphere ; (2) the cells of the so-called annelid cross 

 which alternate with those of (i) and form the cerebral ganglia; 

 (3) the prototroch, forming four groups of cells which constitute the 

 preoral ciliated ring of the trochosphere; and (4) the intermediate 

 girdle cells, forming most of the general ectoderm of the part in front 

 of the prototroch, which is called the umbrella. The expansion of the 

 subumbrellar ectoderm, i.e. that behind the prototroch, is due to the 

 proliferation of a single cell in the second quartette of micromeres, 

 d^ (the somatoblast (Fig. 185, 6)). It forms a plate which spreads from 

 its originally dorsal position round the sides, the two wings uniting 

 behind the mouth to form the ventral plate, becoming the ventral 

 body wall. The descendants of this single cell thus make up nearly 

 the whole of the subumbrellar ectoderm. Its sisters a^, 6^, c"^ give 

 rise to the stomodaeum and are tucked in at the mouth at the close of 

 gastrulation. This marks the completion of the alimentary canal. The 

 young trochosphere now possesses a very thin outer epithelium, 

 thickened in the region of the apical disc and the equatorial ring of 

 cilia, the prototroch, and in the region of the ventral plate, which is 

 the rudiment of a large part of the trunk of the adult worm. It will 

 form ventral nerve cord, chaetal sacs and the ventral and lateral ecto- 

 derm of the trunk. The larval gut opens by a mouth in the equatorial 

 region and consists of an ectodermal oesophagus (stomodaeum) open- 

 ing into the endodermal stomach and an ectodermal hind gut opening 

 to the exterior by an anus. The cavity between the ectoderm and the 

 gut (blastocoele) is spacious and traversed by the pseudopodia-like 

 processes of the mesenchyme cells, larval muscles and nerves, and also 

 contains the two larval nephridia, each of which is composed of two 

 hollow cells placed end to end, one of which contains a "flame" of 

 cilia. They are descended from the first quartette of micromeres and 

 sink in from the surface. 



The trochosphere drifts hither and thither in the sea, swimming 

 feebly by the action of the cilia of the prototroch and sometimes also 

 by secondary rings of cilia (e.g. metatroch formed from cells of the 



