117 



instead, the entire development of the definitive sea star and the conclusion 

 of metamorphosis take place in the purely planktonic bipinnaria stage. Such, 

 in particular, is the development of Luidia ciliaris and L. sarsi (Tattersall and 

 Sheppard, 1934; Wilson, 1978); diwA Astropecten auranciacus and .4. scoparius 

 (Horstadius, 1939; Oguro et al., 1976). The brachiolaria stage in these sea 

 stars is lacking. Hence, on the left side in the late bipinnariae of Luidiidae 

 and Astropectinidae it is possible to see the rudiments of all the organs 

 usually present in the definitive sea star. The organizing center for the for- 

 mation of the definitive organism is located in the region of the middle 

 coelom. We shall discuss metamorphosis in detail below. In other families of 

 sea stars rudiments of definitive organs that serve no functional purpose in the 

 larva are but slightly developed in the bipinnaria stage. These are the five 

 lobate processes of the coelom that appear in the late bipinnaria in the region 

 corresponding to the left hydrocoel, i.e., rudiments of radial canals of the 

 ambulacral system (see Figure 76). At present, based on available data, it 

 cannot be said for certain whether the definitive organs of sea stars lacking 

 in the larva develop during metamorphosis from imaginal "silenf cells, or 

 whether these organs differentiate from larval cells after dedifferentiation. 

 The second course appears more probable (except for gametes, whose fate is 

 predetermined). In the developed bipinnaria of most sea stars, rudiments of 

 temporary organs of the brachiolaria, such as the brachiolar arms and attach- 

 ment disk, appear in the preoral lobe. 



Brachiolaria 



The Brachiolaria differs from the bipinnaria in the presence of attachment 

 organs, large body, and large processes, and pronounced formation of the 

 definitive body of the sea star. 



Feeding, transport of substances, respiration, and excretion : These ac- 

 tivities take place in the brachiolaria in a manner similar to that in the 

 bipinnaria. Barker (1977, 1978) has described the ultrastructure of the 

 integumental epithelium and coelomic lining of the brachiolariae of Stichaster 

 australis and Coscinasterias calamaria. In the region of the brachiolar stalk 

 the cells of the external epithelium contain numerous vacuoles and are armed 

 with microvilli. Under the epithelium lies the plexus of axions underlain by 

 the basement membrane. Under the basement membrane is a connective layer 

 of collagen fibers, fine fibrillar material, and occasional vacuolated cells. The 

 inner basement membrane separates the coelom lining from the connective 

 tissue. The longitudinal muscle fibers adjoin internally the inner basement 

 membrane. The coelomic epithelium lies deepest and comprises mainly flat 

 epithelial cells. Sometimes these cells have pseudopodial processes and long 

 solitary cilia (Figure 78). 



