GEPHYREA. 



1>95 



inwards (fig. 160 B> and give rise to a layer of cells, which appears to 

 extend as an unbroken sheet between the epiblast and bypoblast, and 



FIG. 160. EPIBOLIC GASTRULA OF BONELLIA. (After Spengel.) 



A. Stage when the four hypoblast cells are nearly enclosed. 



B. Stage after the formation of the mesoblast has commenced by an infolding of 

 the lips of the blastopore. 



ep. epiblast; me. mesoblast; bl. blastopore. 



to form the mesoblast. The blastopore now closes up, but its position 

 in relation to the parts of the embryo has not been made out. 



In Phascolosoma (Selenka, No. 369) the ovum, enclosed in a porous 

 zona radiata, divides into two unequal spheres, of which the smaller 

 next divides into two and then into four. An invagination takes 

 place which is intermediate between the embolic and the epibolic types. 

 The small cells, the number of which is increased by additions from 

 the large sphere, divide, and grow round the large sphere. The latter 

 in the meantime also divides, and the cells produced from it form on 

 the one hand a small sack which opens by the blastopore, and on the 

 other they fill up the segmentation cavity, and become the mesoblast 

 and blood corpuscles. The blastopore becomes the permanent mouth. 



Larval forms and development of organs. 



Amongst the Gephyrea inermia the larva has as a rule (Thalas- 

 sema, Echiurus) the characters of a trochosphere, and closely ap- 

 proaches the typical form characteristic of the larva of Polygordius, 

 often known as Loven's larva. In Bonellia this larval form is less 

 perfectly preserved. 



Echiurus. In Echiurus (Salensky, No. 368) the youngest known 

 larva has all the typical trochosphere characters (fig. 161). It is 

 covered with cilia and divided into a prse-oral lobe and post-oral region 

 of nearly equal dimensions. There is a double ciliated ring which 

 separates the two sections of the body as in the larva of Polygordius: 

 the mouth (in) opens between its two elements. The alimentary 

 canal is divided into a stomodseum with a ventral opening, a large 

 stomach, and a short intestine opening by a terminal anus (an). 

 Connecting the oesophagus with the apex of the prae-oral lobe is the 



