234 



METAMORPHOSIS 



Unlike the classes of echinoderms described above, metamorphosis in sea 

 cucumbers is evolutive; the processes of histolysis and resorption during 

 metamorphosis of sea cucumbers are expressed less markedly than in other 

 echinoderms. During this period several larval stages pass in which the ex- 

 ternal shape changes and internal readjustment of larval systems occurs and 

 definitive structures are formed. The planktotrophic, bilaterally symmetric 

 auricularia changes into a nonfeeding planktonic, radially symmetric doliolaria; 

 while living in the plankton, the doliolaria transforms into a pentactula, which 

 is morphologically the closest to the definitive juvenile sea cucumber. The 

 concluding stage of metamorphosis — transition from the pentactula to the 

 definitive individual — occurs after settling on the substrate. The axis of 

 symmetry of the larva and the definitive organism coincides in holothurians 

 (Smiley, 1986). 



Transition of auricularia into doliolaria : In the developed auricularia of 

 Holothuria and Stichopus, before its change into a doliolaria, the so-called 

 elastic spheres are situated under the ciliated band at the ends of the lateral 

 processes (Mortensen, 1937, 1938). Their disposition and number are variable 

 in different species. The function of these structures is not altogether clear. 

 Possibly, their appearance is associated with the transformation of the ciliated 

 band since they are found in the'places where the ciliated band formed in the 

 doliolaria. It may also be assumed that these spheres are particular depots 

 of reserve substances as they disappear earlier than the ciliated bands (Figure 

 183). 



Studies on the development of Stichopus japonicus have revealed that 

 soon after the appearance of elastic spheres, resorption of the lateral body 

 processes begins. In the posterior part of the larva the posterolateral processes 

 come closer while the posterodorsal ones completely disappear. The ciliated 

 band between them reduces and only small areas remain near the elastic 

 spheres. The anal lobe also reduces. The ciliated band disappears completely. 

 The anal opening closes and the intestine shortens. Simultaneous with the 

 changes occurring in the posterior half of the larva, its anterior end is also 

 transformed. The mediodorsal and anterolateral processes disappear. The body 

 becomes flat in the region of the anterodorsal processes. Remains of the 

 ciliated band, as in the posterior part of the larva, are retained only near the 

 elastic spheres. The preoral lobe disappears completely. The preoral processes 

 only shorten. The larva acquires the shape of a barrel with five transverse 

 ciliated bands. These run in the areas of the elastic spheres (Figure 184). In 

 other species of sea cucumbers fragments of the ciliated band of the auricularia 

 play a great part in the formation of the ciliated bands of the doliolaria. Thus, 



