266 



EMBRYOLOGY 



blastopore, the ectoderm becomes invaginated, and forms the 

 oesophagus. This is followed bj an enlargement, the sto- 

 mach of the larva, and this in turn by the small-gut and the 

 hind-gut (Fig. 118 B). 



The formation of the intestine takes place less simply in the cases 

 where the gastrulation is epibolic and the entoderm at first consists of a 

 compact mass of cells. The intestinal wall is only gradually formed, 

 detaches itself from the central yolk-mass, and finally unites at the fore- 

 and hind-guts with the ectoderm (comp. the figures 128 A and B, p. 280, 

 of Psygmobranchvs). 



Trochophore. — Even during gastrulation the embryos 

 rose, with the aid of their vibi-atile apparatus, from the 

 bottom, and betook themselves to the surface of the water. 

 Together with the internal changes described, alterations 

 have also taken place on the outer body, the upper portion 

 of which has become bell-shaped, while the under-portion 



tapers more conically (Figs. 

 118 5 and 119). The band of 

 cilia which lies in front of the 

 mouth extends around the 

 longest periphery. Thereby is 

 reached the larval stage, de- 

 signated by Ray Lankester as 

 the Trochosphere, but now with 

 Hatschek more generally known 

 as the Trochophore. In addition 

 to the organs already mentioned 

 — ciliated band, intestinal tract, 

 and mesodermal bands — still 

 others can be seen in the Tro- 

 chophore. An ectodermal thickening at the upper pole, 

 which bears the tuft of cilia, if such be present, is de- 

 signated as the apical plate ; it represents the fundament 

 of the super-oesophageal ganglion (Figs. 118 A and 119). 

 The cells of the preoral ciliated band also form a similar 

 thickening. These may consist of several successive circles 

 of cells, and between or underneath them is placed a ring 

 of fine nerve fibres, which is connected with ganglionic cells, 

 and is considered by its discoverer, Kleinemberg, as the 



an 



Fig. 119.— Larva of Pofygordius 

 (after Hatschek, from Balfouk's 

 Qo'm'paralive 'Enxhr'\jo\og\f) . an, anal 

 0[)euin<j ; m, mouth-opening ; me. p, 

 mesodermal band ; njifi, head kid- 

 ney ; ol, stomach ; sij, apical plate. 



