229 



mc 



arch 



Figure 176. Cucumaria frondosa 

 (=Psolinus brevis) (Kowalewsky, 1867). 

 Immigration of mesenchyma after 



gastrulation. 



arch — archenteron; bl — blastopore; 



mc — mesenchymal cells. 



Figure 177: Holothuria nobilis (Mortensen, 



1938). 



Late dipleurula. Division of the tubular gut 



into sections. 



e — esophagus; hg — hind gut; 



s — stomach. 



Skeleton : a well-developed larval skeleton is not found in the larvae of 

 holothurians; the skeletal elements are represented by rods of different shapes 

 and small wheels, which are retained in the later stages. These structures are 

 especially characteristic of holothurian auriculariae of the genera Holothuria 

 and Stichopus (Mortensen, 1921, 1937, 1938). 



Feeding : The mechanism of feeding in the holothurian larvae is similar 

 to that in the larvae of other echinoderms. In the early auricularia the preoral 

 lobe above the oral depression has a notch, while the tip of the anal lobe is 

 drawn out somewhat. Between these lobes lies the arched preoral depression 

 (Figure 178). 



The perioral field in holothurian larvae lacks a ciliary cover. The food 

 particles are held in the perioral field by the water currents produced by 

 beating of the cilia in the lateral areas of the band. As in all echinoderms, 

 each cell of the ciliated band bears one cilium. Three to seven cells make up 

 the width of the ciliated band (Strathmann, 1971). On both sides of the 

 ciliated band, numerous secondary cells are present in the auricularia of 

 Parastichopus. 



As the size of the ciliated band bordering the lateral processes increases, 

 the feeding mechanism in the larvae, exhibiting no change whatsoever, be- 

 comes more effective. Besides the single ciliated band, another, the adoral 

 ciliated band, appears around the oral cavity. Cilia are not present in the oral 

 cavity itself and, evidently, because of this, the auricularia has well-devel- 

 oped esophageal muscles and the muscles situated in the dorsal wall of the 

 oral cavity. From the perioral field the food is directed to the esophagus. 



