518 INVEETEBRATA CHAP. 



The larva, rendered heavy by the growth of calcareous matter, 

 sinks to the bottom, and its tube feet adhere to the substratum. 

 The larval arms are rapidly absorbed, beginning with the postero- 

 dorsal ,on the. left side. The ectodermal covering of these arms 

 flows back towards their bases; their supporting rods protrude as 

 naked spines but are soon broken off. The larval stomodaeum 

 becomes disconnected with the stomach and forms a shallow pit; 

 the oral lobe, with the stumps of prae-oral and antero-lateral arms 

 in which this pit lies, persists for a short time but is soon absorbed. 

 The larval anus disappears, but the larval intestine persists. 



At the same time there is a great increase in the mesenchynie, 

 which is budded off principally from the left posterior coelorn, but 

 also from the right, and a corresponding diminution in the still fluid 

 jelly of the blastocoele. The direction right to left in the Echino- 

 pluteus becomes the direction oral-aboral in the young sea-urchin, 

 but this axis is greatly diminished in length as the metamorphosis 

 proceeds, and thus diminution in length must be due to the expulsion 

 of fluid. 



A most remarkable change comes over the staining properties of 

 the ground-substance of the blastocoele at the time of metamorphosis. 

 During the whole of the larval period it is exceedingly difficult to 

 stain, but at the time of metamorphosis that portion of this substance 

 immediately surrounding the stomach becomes capable of taking up 

 stain with avidity. This must be due to the exudation of some 

 proteid substance by the cells of the stomach. This exudation is 

 the forerunner of an exudation which goes on all through life, and 

 constitutes this portion of the blastocoele into the blood system of 

 the adult. It is in reality to be compared witli the lymphatic vessels 

 ensheathing the human intestine, and, in the adult, the connective 

 tissue thus modified, instead of forming a continuous mantle round 

 the stomach, forms a network especially concentrated along two lines 

 forming the so-called dorsal and ventral vessels. 



The imago thus launched on its career is very different from the 

 fully grown adult. It has at first neither open mouth nor anus, 

 for the adult mouth, although already formed, is hidden beneath the 

 epineural veil. The gut forms a simple coil, identical with the 

 curvature of the larval gut. The five azygous tube feet, which 

 terminate the radial canals, are the only organs of locomotion, and 

 the canals extend horizontally outwards; there is no sign of that 

 meridional course so characteristic of the adult urchin. The slightly 

 convex dorsal surface is the rudiment of the insignificant adult peri- 

 proct, but as yet it is greater in extent than the ventral ambulatory 

 surface. In fact, if the long primary tube feet were supported by out- 

 growths of the body we should have a young Asteroid before us. 



There is little doubt that this post-larval stage represents a former 

 Asteroid ancestor. The changes which convert this flat discoid 

 organism into the globular adult are almost of as great importance as 

 those which convert the Echinopluteus into the urchin ; they take 



