522 INVERTEBRATA CHAP. 



the absence of a fixed stage, and in the consequent retention of the 

 larval loconaotor organs till the tube feet become functional. But 

 the Echinopluteus resembles the Asteroid larva, and differs from the 

 Ophiuroid larva in forming the adult mouth on the left side. Now 

 we saw that in all three larvae the larval stomodaeum shallows out 

 and disappears, but that in the Ophiuroid, before this happens, the 

 primary tube feet protrude into the stomodaeum. We may thus 

 interpret the amniotic cavity of the Echinopluteus, into which the 

 tube feet protrude, as a portion of the stomodaeum which is formed 

 separately from the rest ; that is, instead of all the stomodaeum being 

 formed at once and then being twisted to the left, it is formed in two 

 pieces, one of which persists during larval life and then disappears, 

 whilst the other shelters the growing tube feet and then shallows 

 out to form the ectoderm covering the oral surface of the adult. 



ECHINOCYAMUS AND MELLITA 



The development of JZchinocyamus pusillus (Theel, 1892) and of 

 Mellita testudinata resembles that of JZchinus esculentus in all 

 essential points. The calcareous rods supporting the prae-oral arms 

 are, however, in the form of a lattice-work, consisting of three parallel 

 rods connected by cross pieces, and ciliated epaulettes are not formed. 



In Mellita, according to Grave (1902), there is constantly present, 

 in addition to the normal pore-canal, a right pore-canal opening into 

 the right anterior coelom, and the two pore-canals unite to open by a 

 median madreporic pore. 



The larva of Echinocardium cor datum, the development of which we 

 ourselves have studied (1914), develops no ciliated epaulettes, but from 

 the second day of development a club-shaped aboral spike is developed, 

 supported by a lattice-work skeleton consisting of three diverging 

 rods connected by cross pieces. This bears a crest of ciliated 

 epithelium. The skeleton of this club is based on transverse braces 

 connecting up the aboral ends of the recurrent and body rods. The 

 larva also develops horizontally extended postero-lateral arms, which 

 are supported by branches of the skeleton of the aboral spike, and 

 also a pair of antero-dorsal arms supported by branches from the 

 rods supporting the prae-oral arms. 



As metamorphosis approaches the aboral spike is absorbed. A 

 very large number of adult spines are formed before metamorphosis, 

 and the young urchin presents many points of resemblance to a 

 regular urchin. Thus the mouth is in the centre of the aboral 

 surface, and is surrounded by five pointed spines which are very 

 possibly homologous with the teeth of regular urchins. 



EXPERIMENTAL EMBRYOLOGY. ECHINOIDEA 



We must now give a brief account of the principal results 

 obtained by the experiments performed on the eggs of Echinoidea. 



