XVI 



ECHINODEEMATA 



479 



connecting these plates with one another, by means of which the 

 arm can be bent and the ambulacral groove closed, are derived 

 from the cells forming the walls of the perihaemal canals. 



As metamorphosis approaches completion, the septum dividing 

 the peri-oral coelom from the encircling left posterior coelom is largely 

 absorbed, and the two cavities coalesce ; as remnants of this septum 

 there remain ten bands, two in each arm, which constitute the 

 retractor muscles of the adult stomach. The adult mouth is formed 

 by the fusion of the wall of this stomach with the ectoderm ; there 

 is no adult stomodaeum. When the stalk has been almost absorbed 

 the little star-fish wrenches itself loose from the substratum by pulling 

 with its tube feet, and it walks away. 



Fig. 370. — Dorsal {i.e. aboral) views of two yoniig specimens oi Asterina gibbosa shortly 

 after the metamorphosis, {After Ludwig. ) 



A, young star-fish teu days old. B, young star-fish sixteen days old. a, adult anus ; a^.t, azygous 

 tentacle of water-vascular system ; B, hasal plate ; D.C, dorso-central plate ; R, radial plate ; T, terminal 

 plate. In the specimen shown in figure B, only four basals are developed. 



The post-larval development has been followed in Asterina. 

 The main points which have been determined concern the further 

 development of the skeleton and the development of the genital 

 system. We shall deal with the development of the skeleton first. 

 As the arms grow in length new tube feet are added in pairs, the 

 first formed tube feet remaining at the base of the arm, and, 

 alternating with the new tube feet, new ambulacral ossicles are 

 added. At the same time the terminal plates are carried out to the 

 tips of the arms, and new plates are intercalated between them 

 and the central plate. The most important of these, and the first 

 to appear, are the radials situated at the bases of the arms. The 

 names basals, dorso-central, and radials, it may be remarked, have 

 been bestowed on these plates from a suggested homology with the 



