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IDENTIFICATION OF PELAGIC LARVAE OF SEA CUCUMBERS 

 (Terminology and Diagnostic Characters) 



The absence of a developed endoskeleton is characteristic of holothurian 

 larvae. The larvae develop through several stages differing in external mor- 

 phological features, which are important for their identification. 



Species with planktotrophic larvae pass through the stages of dipleurula, 

 auricularia, doliolaria, and pentactula. Species with lecithotrophic larvae pass 

 through the stages of doliolaria and pentactula. The species characters at the 

 stage of dipleurula are very poorly developed and cannot be used for the 

 identification of larvae. The latter becomes practically possible only from the 

 stage of auricularia, when lateral processes appear. There are generally six 

 pairs of processes differing in size but sometimes some of them do not 

 develop at all. Their position in a fully formed larva is as follows: above the 

 preoral lobe lie the anterodorsal processes, followed by the mediodorsal, and 

 the preoral process behind them. Somewhat lower are the postoral processes, 

 below them the posterodorsal, and at the base of the anal lobe the postero- 

 lateral processes. The auricularia has a single ciliated band. The lower end 

 of its body may be triangular, as in the larvae of the genus Holothuria, or flat, 

 as in the genus Stichopus. At this stage, the larvae of some sea cucumbers 

 have various spicules in the form of simple calcite plates of different shapes 

 or in the form of wheels and hooks. These skeletal elements are retained in 

 the later stages of larval development. In some members of the genera 

 Holothuria and Stichopus elastic spheres appear in the auricularia stage at the 

 tips of the processes and sometimes at the base of the larval body {Holothuria). 

 Their number and arrangement vary in different species. These spheres also 

 persist in later stages of development. 



The doliolaria is barrel-shaped with four— five transverse ciliated bands 

 that are isolated from each other. 



The lecithotrophic doliolaria differs in that the preoral and anal fields do 

 not shed their ciliary cover. The larva has no skeletal elements and elastic 

 spheres and the ciliated bands range from two to five. 



The pentactula has well-developed primary tentacles, which may be di- 

 stally bifurcate or bear numerous papillae of various shapes. Rods, the rudi- 

 ments of definitive ossicles, appear at this stage. In larvae with a planktotrophic 

 type of development the ossicles formed in the auricularia stage and the 

 ciliated bands are retained. 



For identification of planktotrophic larvae, of primary importance are the 

 structure of the skeletal elements, the presence and number of elastic spheres 

 and their arrangement, and the degree of development of the lateral processes 

 in the auricularia. 



