214 LECTURE X. 



cavity of the body, and so reach the genital pores on the ventral 

 surface. The Echinaster cannot have these pores on the dorsal 

 surface (as Miiller describes in the Aster acanthion rubens and Solas- 

 ter papposus), because the ova would fall into the sea, and not into 

 the ventral concavity, into which they are received, as into a mar- 

 supium, and are hatched there. On the 17th March, Sars found the 

 star-fish strongly contracted, and the disc raised into a dome shape, 

 the mouth and its surrounding parts forming the pouch, in which the 

 ova are hatched, and the young remain some time. Some embryos 

 were one and a half line, others three-fourths of a line, in size : the 

 ova and the monadiform young are loose in the concavity ; but the 

 further developed young are attached by a special pedicle to the 

 walls of this improvised marsupium. 



The impregnated eggs have the chorion separated from the yolk 

 or germ-mass by a clear fluid. In the excluded ova the germinal 

 vesicle had disappeared, and the yolk was seen in different stages of 

 division. On the 7tli March, at 9 a.m., the yolk was seen in two 

 hemispherical, not quite distinct parts. In the evening each of 

 these were similarly divided, forming four lobes. At ten the next 

 morning, each lobe had again divided, and thus eight spheroids were 

 seen. In the evening of the 8th all the surface of the germ-mass 

 resembled a mulberry. On the 17th March the ciliated embryos 

 were seen, oval, blood-red, swimming about the marsupial chamber. 

 They first develop two tubercles, then four tubercles, which are 

 ciliated, and they swim, rotifer-like, with these forwards. After- 

 wards the body becomes depressed. The next, which may be called 

 the pentacrinal or polype, stage, was noticed on the 3rd April, when 

 the embryo star-fishes presented an organ of adhesion or pedicle, well 

 developed ; it is short, cylindrical, with thick round ends ; four 

 shorter processes then bud out, and a fifth much smaller tubercle. 



The smooth dorsal surface, and the ventral surface of the flattened 

 body, may be now recognised, with five double rows of small, clear 

 tubercles, radiating from the centre : these become the ambulacral 

 tentacles. The young attach themselves, like crinoids, by their 

 quadrifid pedicle to the side of the glass vessel into which they may 

 be put. When detached, they still can swim round the bottom of 

 the glass by the cilia covering the pedicle and body, with the pedicle 

 foremost, and the belly upwards. When turned with the back 

 upwards they lay still. The whole yolk is here converted into the 

 germ-mass, and then into the embryo. 



On the 15th April, five corners began to grow out of the periphery 

 of the depressed body. Ambulacral tentacles protrude from the 

 tubercles. At the under surface and outer end of each of the arms 



