478 



INVERTEBRATA 



CHAP. 



B 



m 



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

 Asterina gibbosa eight days old, to show the 

 progress of metamorphosis. (After Ludwig. ) 



A, from the right side. B, from the left side. Letters 

 as in previous figure. In addition, t.f, rudiments of 

 paired tube feet. 



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 

 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 

 of the terminal plates (T, 

 Fig. 370), which overarch and 

 protect the azygous tentacles 

 in which the primary 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 of 



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 



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 appear 



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



spicules are the rudiments of the ambulacra! plates. The muscles 



Longitudinal frontal section through 

 a larva of Asterina gibbosa about the same 

 age as those shown in previous figure. 

 (Original.) 



Letters as in Figs. 365 and 3C6. In addition, aM, 

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

 caecum. 



