XVI 



ECHINODEEMATA 



531 



plate {aj), Fig.. 395). _ At the opposite pole the invagination which is 

 to form the archenteron appears. This is of very small extent in com- 

 parison with the length of the blastula ; it does not, at the period of 

 its greatest extension, equal in length half the total length of the 

 embryo. 



When the development has reached this point the embryo escapes 

 from the egg-capsule and begins to lead a free-swimming life. In 

 Synapta, therefore, the larval life commences with the gastrula stage, 

 not with the blastula as in the other groups studied. 



The mesenchyme is formed only after the gastrula stage has been 

 reached. It originates, according to Selenka, in two cells given off 

 from the apex of the archen- 

 teron which multiply by 

 division. This is unlikely, it 



more probable that 

 numerous cells are given off'. 



arch 



Fig. 394. —The 32-cell stage in the 

 segmentation of the egg of Synapta 

 viewed from the side. 



(After Selenka ) 



]?io. 395. — The free-swimming gastrula of 

 Synapta digitata. (After Selenka. ) 



ap, apical thickening ; arch, archenteron ; Up, blastopore. 



In, the late origin of the mesenchyme the Synapta larva resembles the 

 Asteroid, and the reason for this may be the same in both cases, 

 namely, the absence of a larval skeleton, to the formation of which 

 is devoted the primary mesenchyme of Ophiuroids and Echinoids, 

 which is given off in the blastula stage. 



The tip of the archenteron bends at right angles and grows 

 towards the dorsal side of the larva ; it fuses there with the ectoderm, 

 and a perforation of the fused layers is effected so that the lumen of 

 the archenteron communicates with the exterior. This perforation is 

 the madreporic pore {mp, Fig. 396). After it is formed the horizontal 

 branch of the archenteron is separated from the vertical one and 

 becomes the coelom, whilst the vertical section of the archenteron is 

 the gut. This latter soon becomes divided by constrictions 



