138 



PHYLUM ECHINODERMATA. 



these internal changes it will be well to consider briefly the 

 various forms which the bilateral larva may assume. 



The auricularia larva. In the Holothurians the ciliated band 

 becomes elongated and sinuous, as is shown in the series of 

 diagrams in Fig. 90. A completely developed auricularia larva 

 of a Holothurian is shown in Fig. 91. The sides of the body 

 are prolonged into processes which correspond to the arms of 

 the bipinnaria and pluteus, and there is a well-marked preoral 

 lobe. Calcareous structures in the form of spheres, wheels and 

 star-shaped bodies are formed, but there are no calcareous rods. 

 In some species (Auricularia stelligera and sphaerigera) peculiar 

 elastic spheres, the nature of which is not known, are present. 



5 ' 



FIG. 91. Auricularia stelligera, 

 ventral view (after J. Miiller). 

 1 frontal area ; 2 preoral pro- 

 cess ; 3 anterior, 4 posterior 

 portion of the ciliated band ; 

 5 postoral process ; 6 anal 

 area ; 7 postero-lateral pro- 

 cess ; 8 postero-dorsal pro- 

 cess ; 9 oral depression ; 10 

 dorso-median process ; 11 

 antero-dorsal process. 



FIG. 92. Pupal stage of the larva 

 of Synapta digitata (from Kor- 

 schelt and Heider, after Se- 

 mon). ed hind-gut ; ent right 

 body- cavity ; m oral funnel ; 

 to water-vascular ring, with 

 out-growths into tentacles t 

 and radial vessels p. 



The larva now enters the so-called pupa-stage, in which it has 

 the form of a barrel with five ciliated hoop-like bands (Fig. 92). 

 The pupa is formed from the auricularia in the following way : 

 The ciliated ring breaks up (Fig. 93) into pieces which rearrange 

 themselves into the five rings of the pupa and the oral ring ; 

 the mouth and surrounding parts, including the oral ring, retreat 

 into the interior, giving rise to the oral vestibule (atrial cavity), 

 the opening of which narrows and passes to the left side. Event- 

 ually the atrial opening becomes terminal by the atrophy of 

 he small preoral lobe, and the epithelium of the oral ring 



