216 



Experiments on embryos reared in vitro showed that for their normal 

 development additional substances are required. Observing the morphological 

 changes in the bursal wall in the last stages of development of embryos. Fell 

 concluded that nutrients are produced by the wall itself and feeding of the 

 embryo occurs by absorption through the entire surface of the embryo. 



Another case of true viviparity has been described by Mortensen (1921) 

 in the Antarctic brittle star Ophionotus hexactis. In this species only one egg 

 matures in the ovary; development of the juvenile is also introovarian, evi- 

 dently due to feeding nutritive and visceral fluid (Turner and Dearborn, 

 1979). After emergence of the young brittle star, a new ovary develops. 



Hyman (1955), based on data on the reproduction of brittle stars, con- 

 cluded that in the viviparous species the spawning period is more protracted 

 than in the species with lecithotrophic and planktotrophic development. A list 

 of species and their mode of development is given in Table 6. 



IDENTIFICATION OF PELAGIC LARVAE OF 



BRITTLE STARS 



(Terminology and Diagnostic Characters) 



Brittle star larvae have an endoskeleton composed of rods that support the 

 arms and basal part of the body, which is flattened dorsoventrally. The arms 

 are listed in the order of their appearance. In the early stages of plutei the 

 posterolateral and anterolateral arms develop on the ventral side and later the 

 postoral arms develop on the dorsal side. Generally, the rods supporting the 

 arms are simple, but in some species the rods of the posterolateral arms may 

 be fenestrated. The basal rods passing in the body of the larva may be distally 

 bifiarcate and sometimes the ends are interlocked. In many species one or two 

 pairs of transverse processes develop in the lower third of the basal rods. 

 Another pair of arms, the posterodorsal, appears at a later stage; these arms 

 develop behind the anterolateral arms on the ventral side. Their supporting 

 rods appear as processes of the anterolateral rods. Between the basic arms, 

 except the anterolateral, vibratile lobes may develop. Pigment granules ap- 

 pear under the ciliated band at the tips of the arms and in the region of the 

 stomach. Pigmentation varies in different species. 



The following diagnostic characters are usefiil for identification of brittle 

 star larvae. 



1. Number of arms. Three pairs in the first stage and four pairs in the second 

 stage. 



2. Structure of rods of the larval skeleton. In larvae of the family amphiuridae 

 all skeletal rods are simple. 



