PROTECTED DEVELOPMENT. 285 



The formation of the embryo takes place thus : four shield-like 

 thickenings are formed upon one segment of the ovum, and beneath 

 them several ambulacra 1 feet make their appearance. The star is 

 developed by the increase in size and number of these discs and 

 ambulacral feet. At this period of development we can distinguish 

 the circular ambulacral vessel surrounding a central hemispherical 

 projection of the oral disc, also the five vascular trunks and '2 3 

 pairs of sucking feet in each ray. In other cases, the brood pouch 

 is formed upon the ventral surface of the Star-fish, e.g., Echinaster 

 Sarsii, and the embryo, which is completely ciliated, is provided 

 at the anterior end with a knobbed process. The latter is divided 

 into several structures (Haftzapfchen), which serve as organs to 

 attach the body of the embryo to the wall of the brood pouch. 

 Suctorial feet are now formed in each ray, two paired and one 

 unpaired, the latter lying nearest to the angle of the pentagon. 

 The five angles come to project more and more, and acquire 

 eye spots and ambulacral grooves. Spicules appear, and the 

 mouth perforation is formed, the fixing organ aborts, and the em- 

 bryos escape from the maternal brood pouch ; and being at this 

 time capable of creeping and of nourishing themselves independently, 

 they gradually develop into small star-fishes. The mode of deve- 

 lopment is the same in Asteracanthion Mullerii, and some Ophiurids, 

 as Amphiura squamata. 



Amongst the Holothurids (//. tremula) the simple and more direct 

 development was first observed by Danielssen and Koren, and later 

 by Kowalevski, in Phyllophorus urna, and by Selenka, in Cucumaria 

 doliolum. In the first case the embryo leaves the egg in the form of 

 a ciliated larva, which soon assumes a pear shape, and develops the 

 circular vessel of the water-vascular system, and five tentacles round 

 the mouth. The alimentary canal and the dermal skeleton make 

 their appearance before the five tentacles have assumed the function 

 of locomotion in place of the cilia which have disappeared. Later on, 

 with the progressive growth, the tentacles become branched, and two 

 ventral feet appear, which put the bilateral symmetry of the larva 

 beyond all doubt. In all cases, even in the cases of a more direct 

 development, the radial symmetry of the adult Echinoderm appears to 

 be preceded by a bilateral symmetry of the larva. 



All the Echinoderms are inhabitants of the sea ; they are capable 

 of a slow, creeping movement, and feed on marine animals, especially 

 on Mollusca, but also on Fuel and sea-weeds. Some are found near 

 the coast at the bottom of the sea, others occur at considerable 



