xvi ECHINODERMATA 473 



coelom. The larval intestine is straight, and both it and the larval 

 anus disappear shortly after the animal enters on its larval existence ; 

 that is on the fifth day. The coelom is only separated from the gut 

 after the stomodaeum has broken through. 



As for the second point (6), the larva has the form of a boot. The 

 sole of the boot is the prae-oral lobe, which is enormous, and the 

 " upper " of the boot is the body. The back of the boot corresponds 

 to the ventral surface of the larva, and here the larval mouth is 

 situated ; while the front of the boot is the dorsal surface. The 

 prae-oral lobe is surrounded by a thickened ridge which bears 

 specially long cilia, by the aid of which, and of the cilia of lesser 

 length which cover the ectoderm everywhere, the larva glides about 

 on the bottom. It uses the prae-oral lobe as a sucker, attaching the 

 thickened rim to the substratum on which it is moving, and then 

 retracting the centre. In the centre there appears, on the seventh day, 



A B 



coe 



FIG. 363. Frontal longitudinal sections of two early embryos of Asterina gibbosa, 

 in order to show the development of the coelom. (Original.) 



A, stage in which the coelom is a spherical vesicle. B, stage in which the coelom is growing liack 

 in tongues at the sides of the gut. <tl, rudiment of gut : Wj>, blastopore ; coe, rudiment of the coelom. 



a circular area of thickened glandular epithelium (Fig. 364). This is 

 the fixing disc, and by means of the secretion produced by it the larva 

 effects a permanent fixation to the bottom. Once this has been 

 accomplished the rim is destroyed by the same process as that by 

 which the brachiolar arms are removed in the Bipinnaria larva. The 

 whole prae-oral lobe shrinks, until final atrophy takes place and the 

 larva wrenches itself free and walks away as a little star-fish. 



The internal changes which take place during the larval life, and 

 the metamorphoses, are known in detail. Let us go back to the time 

 when the transverse septa are found in the coelom. The septum is 

 formed on the left side before it is formed on the right, and in both 

 cases it begins at the dorsal side and grows down to the ventral 

 surface. On the left side, the septum, after formation, becomes 

 perforated by two holes, a dorsal and a ventral one. In this way a free 

 passage of fluid between anterior and posterior coeloms is allowed ; 

 and as the cells of the coelomic wall, as in the Bipinnaria larva, 

 secrete muscular fibrils, and the larva can change its shape very much, 



