VIII 



ECHINODERMATA—GABE OF BROOD 



503 



this is only tlie case in the female, we here have a, striking sexual dimorphism. 

 Similar adaptations for the care of the brood seem to occur in Jlloira, Anochamis, 

 etc. 



(c) Asteroidea. — Among the Asteroidea, the Pterasteriiiai {Pterasfer, Symenastcr) 

 are very remarkable for the care of the brood. The whole of the apical body wall 

 carries large peculiarly shaped paxillse or calcareous pillars, from the free ends of 

 which radiate, like the spokes of a wheel, a varying number of calcareous rods {cf. 

 p. 391). All these calcareous stars of the paxillfe are connected by an integument 

 in such a way that, between this in- 

 tegument (supradorsal membrane) and 

 the dorsal wall of the body beneath it, 

 a large brood chamber is formed. This 

 chamber communicates with the ex- 

 terior at many points : (1) through a 

 large aperture at the apical pole 

 (osoulum) usually surrounded by five 

 valves of considerable size (Fig. 396) ; 

 (2) through numerous contractile pores 

 or spiracles in the membrane which 

 covers the brood cavity ; (3) through 

 regular segmentally recurring apertures 

 at the sides of the aims. These aper- 

 tures can be closed by means of small 

 spines or scales. These "segmental" 

 apertures are regarded by the present 

 writer as ventilating apertures, as they 

 appear to serve the purpose of keeping 

 up an active circulation of water in the 

 brood cavity. 



The sexual arrangements in the 

 Pterasterince are unfortunately still un- 

 known. All specimens as yet desc:ibed 

 show the brood membrane. Possibly they are all females, and the males are still 

 unknown, or the Pterasterince may be hermaphrodite. Or, again, there may be a 

 far-reaching dimorphism, which has led to the males being described as a separate 

 species. 



Leptoptycliaster kergicelencnsis, an Astropeetinid, shows us the care of the brood, 

 seen in the Pterasterhue, to a certain extent in statu nascendi. The eggs which 

 emerge from the genital aperture pass into the interstices between the stalks of the 

 still separate paxilliu, and there pass through the first stages of their development. 

 At a later stage, also, as young Asteroids, they remain for some time on the body of 

 the mother. 



In Asterias spirabilis, similar arrangements are found, but the embryo is con- 

 nected by means of a stalk to the body wall of the mother. 



Other Asteroids (e.g. species of Echinaster and Asterias) protect the brood which 

 collects on the oral side ; it develops under the shelter of the arms, which simply 

 bend round over it, so forming a temporary brood chamber. 



Ophiuroidea. — In the description of the bursas, p. 495, it was mentioned that, in 

 many Ophiurids, these function as brood chambers, and the best -known cases were 

 given. 



Fig. 3i)6.— Hymenasterpellucidus, Wyv. Thom- 

 son, from the apical side. The osculum is seen, 

 surrounded by iive valves (after Sladen). 



