478 



INVEKTEBKATA 



CHAP. 



Asterina, and in all other Echinoderms which have been studied, 

 are little triradiate spicules embedded in and produced by the 

 mesenchyme cells intervening between coelomic wall and ectoderm ; 

 each arm of the spicule, as it grows, bifurcates, and the forks of 



adjacent arms join one 



A B another, and in this way a 



mesh is formed. From the 

 junction of the two forks 

 another arm is given off, 

 and, by a repetition of the 

 processes of forking and of 

 union of forks, a network of 

 calcareous meshes is slowly 

 built up. 



The first spicules to 

 appear in Asterias and 

 Asterina are the rudiments 



Asterina gibbosa eight days old, to show the of the terminal platCS (T, 



T T T \ ^ ^ 



IE 



FIG. 368. Views from the side of a larva of 



Fig. 370), which overarch and 



progress of metamorphosis. (After Ludwig.) 



A, from the right side. B, from the left side. Letters protect the azygOUS tentacles 

 as in previous figure. In addition, t.f, rudiments of i i j-i it, 



paired tube feet. m whlch the P^aiy lobes 



of the hydrocoele, or radial 



water- vascular canals, terminate. Alternating with these terminal 

 plates arise five basal plates 

 (B, Fig. 370), one of which 

 surrounds the madreporic pore 

 and is the rudiment of the 

 madreporite of the adult^. 

 In the centre of the circle ol 

 basals there arises the so-called 

 dorso-central plate (D.C, Fig. 

 370). This plate does not lie 

 over the right posterior coelom 

 but rather to one side of it, 

 and the adult anus, which 

 appears at one side of the 

 dorso-central plate, is conse- 

 quently situated over the 

 mesentery separating the left 

 posterior and right posterior FlG 369- _ Longitudin!ll frontal section tlirough 



a larva of Asterina gibbosa about the same 

 age as those shown in previous figure. 

 (Original.) 



coeloms. 



The rectum, which is 

 formed as an outgrowth from 

 the larval stomach, lies in this 

 mesentery. On the ventral 

 side of the disc there appeal- 

 pairs of spicules alternating with the rudiments of tube feet. These 

 spicules are the rudiments of the ambulacral plates. The muscles 



Letters as in Figs. 365 and 300. Tn addition, ale 1 , 

 rudiment of axial sinus ; py.r, rudiment of pyloric 

 caecum. 



