2S4 



ECHIXODERMATA. 



of the stomach and the circular vessel, which is formed from the 

 dorsal tube. They are at first enclosed in a cavity, from, which later 



on they penetrate to the exterior. 

 The larva retracts its lateral lobes 

 and transforms itself into a barrel- 

 shaped body with five transverse 

 rows of cilia, and loses the mouth 

 and dorsal pore (fig. 229). The 

 ambulacra! system gradually de- 

 velops further, the intestine be- 

 comes longer, the first five tentacles 

 break through to the exterior, the 

 mouth appears at the anterior pole, 

 and the first suctorial foot with its 

 ambulacra! vessel is seen on the 

 ventral surface (fig. 230). The 

 animal gradually loses the bands of 

 cilia, and as a young Ilolothurian 

 creeps about by means of its ten- 

 tacles and of the first ambulacral 

 foot, which is soon followed by a 

 second new one. 



In the more direct development the 

 bilateral larva seems to be more or less completely suppressed, and 

 the time of free-swimming life shortened or altogether dispensed with. 

 In these cases, protective 

 arrangements, such as brood 

 pouches, are always present 

 in the mother. The brood 

 pouch of Pterastcr militaris 

 is the most carefully pro- 

 tected. It lies above the 

 anus and generative open- 

 ings ; its walls are highly 

 charged with calcareous 



FIG. 229. Auricularia pupa of Synapta 

 seen in profile (after E. Metschnikoff). 

 The mouth is already large, so that 

 the tentacles (T) can be protruded. 

 TfV, Ring of cilia; Pe, Pi, somatic 

 a.nd visceral layers of the peritoneal 

 sacs; Ob, auditory vesicle; Po, pore 

 of the water-vascular system; It, cal. 

 cureous wheel-shaped body. 



FIG. 230. Young Holothurid with extended ten- 

 tacles (T), swimming and creeping (after J. 

 Mailer). 



matter, and they are raised 



above the spicules on the 



back. From eight to twenty 



ova (1 mm. in diameter) pass into the interior of the brood pouch, 



and are there developed into oval embryos, which acquire several 



sucking feet and assume later the form of a star with five rays. 



