XVI ECHINODEEMATA 501 



The genital rachis grows out from the peripheral end of the 

 stolon, and the aboral sinus surrounding it arises by the development 

 of a covering flap, just as happens in Asteroidea. The genital rachis 

 pursues an undulating course, being situated aborally where it crosses 

 the radii and ventrally in the interradii. As it slopes downward 

 from radius to interradius it passes over the surface of the genital 

 bursae, which develop as invaginations of the ectoderm. Here the 

 genital organs develop as buds, which press against the bursa and 

 form their own openings into it at maturity. Thus all the parts 

 which can be recognized in the genital system of Asteroids are repre- 

 sented in Ophiuroids, but the aboral end of the stone-canal has been 

 rotated downwards (Fig. 381). 



• 



OPHIUKA BKEVISPINA 



The development of Ophiura brevispina (1900) has been worked 

 out by Caswell Grave. The egg is comparatively large and very 

 yolky ; and development is rapid, the metamorphosis being complete 

 in ten days. The development bears somewhat the same relation to 

 that of Ophiothrix as that of Solaster does to Asterias. The earliest 

 stages were not observed, but it appears that a blastula is formed, the 

 cavity of which becomes filled with precociously formed mesenchyme. 

 Then an invagination of a solid mass of cells takes place, which 

 becomes hollowed out so as to form an archenteron, in such a way 

 that, for a time, a solid tongue of cells projects into its cavity. 



The gastrula rapidly elongates, and there is formed in front of the 

 archenteron a long head region filled with mesenchyme. From the 

 apex of the archenteron a coelomic vesicle is cut off, which soon 

 divides into right and left sacs ; these form the right and left anterior 

 coeloms. At the same time the rest of the archenteron becomes cut 

 by a circular furrow into a small dorsal sac, which forms the stomach, 

 and large ventral one, the front part of which forms the hydrocoele 

 and the hinder part the posterior coelom. The mouth arises as a 

 wide shallow invagination on the ventral surface, while the hydro- 

 coele rapidly develops its five lobes and grows round it. 



The external form of the larva is now cylindrical, but slightly 

 thicker behind than in front. The universal coat of cilia has given 

 place to five transverse rings of cilia, which give the creature the 

 appearance of being segmented and led Johannes Miiller, by whom it 

 was first discovered, to call it the "worm-like larva." It glides 

 slowly over the bottom. Soon the hydrocoele ring becomes completed 

 and the azygous tentacles appear as external protrusions, and in this 

 way a pentagonal figure is outlined on the under side of the posterior 

 part of the larva. 



Although the hydrocoele arises from a rudiment common to the left 

 posterior coelom, it becomes cut off from this and forms a secondary 

 connection with the left anterior coelom, which is the stone-canal. 

 Only after this has occurred is the pore-canal formed. The larva now 



