221 



OPHIURA SARSI LUTKEN 



(Ophiuridae) 



Mature eggs are reddish, nontransparent, 100 |im in diameter. Under labora- 

 tory conditions the 8-cell stage is formed 1 hr after fertilization at a tempera- 

 ture of 16°C. After 6 hrs, the blastula can be seen; 2 hrs later the blastula 

 begins to swim but hatching from the membrane does not occur. Cilia pass 

 through the membrane adjoining the outer wall of the blastula. At this stage, 

 the blastula is spherical. After 21 hrs, the blastula changes, elongating along 

 the animal-vegetal axis; the animal pole is slightly tapered and the vegetal 

 pole flattened. The gastrula with a well-developed archenteron is formed 

 after 45 hours. An accumulation of primary mesenchymal cells is observed 

 around the base of the archenteron. In shape the gastrula resembles an isos- 

 celes triangle whose base is situated at the vegetal pole. The next day, 

 tetraradiate calcite spicules — rudiments of the larval skeleton — are discern- 

 ible among the cells of the primary mesenchyme. The length of the embryo 

 at this stage is 150 |im. Along the perioral depression a ciliated band is 

 formed; all other cilia, except in the basal part of the body, disappear. Pluteus 

 I stage is fijlly formed on the 6th day. 



Pluteus I Stage 



At the point of divergence of the basal rods, the larval body has a distinct 

 girdle. All the skeletal rods are simple and provided with fme spinules. The 

 posterolateral arms are the first to differentiate, followed by the anterolateral 

 arms. The preoral arms remain for some time in a rudimentary state. As the 

 pluteus grows, the structure of the basal rods becomes more and more com- 

 plex. Large spines, two on each rod, arise on their distal ends, they contact 

 to form a ring. The ends of the basal rods, initially pointed, now become 

 thickened and flattened. The yolk membrane, yolk granules, and cilia disap- 

 pear at the basal end of the pluteus. By the time rudiments of the fourth pair 

 of arms appear, the pluteus has reached 450 fim in length; 550 |im between 

 tips of the posterolateral arms (Figure 171). 



Pluteus II Stage 



Plutei of this stage are distinguished by the yellow color of the stomach. 

 The fourth pair of rods, the posterodorsal, forms as processes of the anterolateral 

 rods, which are found in their lower third (Figure 171). On full development, 

 these rods are slightly shorter than the postoral. As soon as the larva reaches 

 a length of 750 |im and 1,250 )j.m between the tips of the posterolateral arms, 

 the skeletal rods become yellowish-brown and the stomach becomes green. 

 The basal rods are distally form thick processes, by means of which the rods 



