KCHIXODERMATA. 213 



developed ; and thus is arranged the combination of male with female 

 parts, which w^e shall find not uncommon in the hermaphrodite 

 mollusks. In all other known Echinoderms, the male organs are 

 peculiar to one individual, and the female organs to another ; but the 

 organs are very much alike in both sexes, and, with the modifications 

 which have already been described, are, upon the whole, simple in 

 their structure. 



With regard to the development of the typical star-fishes, we are 

 indebted for our first and chief information to Sars.* This con- 

 scientious observer found, in the month of March, great numbers of 

 a small kind of star-fish crouching under the rocks at low water, of 

 different forms, but generally with their disc forming a prominent 

 dome above them. The ova, as they become matured in this species, 

 assume a comparatively large size, break out of the ovisacs, and 

 escape by the genital apertures near the mouth. The ova are of a 

 bright red colour, and are received and incubated, as it were, in the 

 hollow of the dome-shaped disc ; and in them Sars traced almost 

 every stage in the development of the species. He commenced his 

 observations in the month of February, 1840, at Floroe, in Nor- 

 way, on the species called Echinaster sanguinolentus. This has ten 

 ovaria, usually whitish, then reddish, as the ova are matured. In 

 these the chorion is strong and elastic, the yolk finely granular; 

 there is no white or albumen. The nucleated germinal vesicle is 

 large and distinct, and usually escapes entire when the q^% is burst 

 under compression. The ovaria are grape-like bunches of pyriform 

 sacs, with ova of different sizes : they being progressively developed. 

 In each ovarium two or three eggs were as large as all the rest put 

 together. The thin, colourless chorion immediately invests the yolk, 

 which is blood-red and opaque in the big eggs : in the small ones it 

 is lighter coloured and clear, permitting the germinal vesicle and 

 nucleus to be seen. In March he found seven or eight large eggs in 

 each ovarium. 



How they escape was not seen ; the genital pores were too small 

 to be clearly discerned. Tiedemann thinks the eggs escape by the 

 pores above the five dentated processes of the mouth, and Sars thinks 

 so likewise ; because he once, by pressure on the body of the star- 

 fish, saw a red slimy filament, like the yolk of squashed eggs, come 

 out of a little hole near the angle of each ray, above the mouth, and 

 because the ripe eggs are always at the upper or free end of the 

 ovarium, and not likely to traverse the stem or radius of the oviduct 

 which he deems a tendinous band. Most likely they fall into the 



* CLXIV. and CLL p. 47., pi. 8. 

 !• 3 



