203 



takes place through contraction of the muscles — dilatators — of the mouth, 

 which extend from the ends of the preoral transverse band to the anterolateral 

 arms. These muscles are able to contract together or individually. Expulsion 

 of algae from the oral cavity is effected only by raising the preoral band 

 through contraction of these muscles as well as contraction of the muscles of 

 the dorsal wall of the oral cavity, without involvement of the esophageal 

 muscles (Strathmarm, 1971). 



Respiration, transport of metabolites, and excretion: In brittle star larvae 

 there are no special organs of respiration and excretion. The hemal system is 

 also absent. Transport of nutrients is, possibly, done by the fluid of the 

 primary and secondary body cavities. 



At this stage, the coelomic sac is still not connected with the surrounding 

 medium. The primary coelom is elongated and the two parts connected by a 

 narrow canal are readily discernible. The left coelomic sac is divided into 

 two: the upper — axocoel and hydrocoel, and the lower — somatocoel (Fig- 

 ure 155). Such a division was observed by McBride (1907) in Ophiothrix 

 fragilis and by Grave (1898) in Ophiocoma echinata. Olsen (1942) found that 

 in Ophiopholis aculeata the axocoel is the first to separate from the primary 

 coelom. 



and 



Figure 155: Ophiothrix fragilis (McBride, 1907). 



Separation of the somatocoel from the anterior section of the coelom. 



and — left anterior coelom; ancr — right anterior coelom; Is — left somatocoel; rs — right 



somatocoel. 

 Remaining legend same as in Figure 154. 



