169 



tracts of the adult sea urchin Dendraster excentricus (Burke, 1984, 1986). 

 Probably, this protein serves as the active agent at induction of metamorpho- 

 sis in larvae using water from the aquarium in which the adult individuals are 

 held; in nature, or using sand from natural habitats (Highsmith, 1982). Even 

 before some of this corroborative data was available, Stratlimann (1978) had 

 concluded that the primary podia ought to perform the function of attachment 

 and consequently possess secretory and sensory cells capable of receiving 

 mechanical and chemical stimuli. In 1980, Burke investigated the morphol- 

 ogy of the primary podia of urchins and demonstrated that sensory and secre- 

 tory cells actually are present on the surface of the suction disks of the podia. 

 Sensory cells occur over the entire surface of each disk and lie in groups of 

 3 — 5 cells along its margin. Apically, each cell bears a simple cilium. The 

 basal ends of the sensory cells taper to the axonal process, which constitutes 

 the subepithelial nerve plexus (Figure 125). Thus, the presence of sensory and 

 secretory cells in the attachment of primary podia disks plays an important 

 role in scouting the substrate, settling, and successftil completion of metamor- 

 phosis of the urchin. 



LECITHOTROPHIC LARVAE 



Among the present-day "regular" sea urchins only a few species exhibit brood 

 care. They are representatives of the order Cidaroida, including Austrocidaris 

 canaliculata (Hyman, 1955), Ctenocidaris nutrix, and Stereocidaris nutrix 

 (Baranova, 1968) from the Antarctic waters. They have special brood cham- 

 bers — dermal outgrowths — situated in the buccal field or the periproct. The 

 developed young is later transferred to the aboral side of the test where it 

 remains for some time before undertaking independent existence. Besides 

 cidaris, Hyrsiechinus coronatus (Mortensen, 1903) seems to be the only known 

 regular urchin with brood care. The young in this species are transferred to 

 the apical system of plates around the raised periproct. 



In "irregular" urchins, the brood chambers are situated in the female 

 petaloids. Such urchins, are Fibularia nutriens (Mortensen, 1948; Hyman, 

 1955) from the order Clypeasteroida (sand dollars), which have a small in- 

 flated test and are found in the Pacific. Among the heart urchins (order 

 Spatangoida), there are also species in which the young develop in brood 

 chambers. One such urchin, Abatus (Hemiaster) cavernosus (Meisenheimer, 

 1921; Kilias, 1969), occurs around Kerguelen Island in the Southern Indian 

 Ocean. 



The presence of brood chambers is usually observed in species living 

 either in Antarctic waters or at great depths where the water temperature is 

 low. Yet, in some sea urchins inhabiting waters of low temperatures, brood 

 chambers are absent. This is observed in Pourtalesia jeffreysi (order 



