502 COMPARATIVE ANATOMY CHAP. 



Eggs as well as spermatozoa are produced in the gonadial tubes, but the two 

 products do not ripen simultaneously (Synapta inhccrens). The spermatozoa only 

 ripen after the ejection of the eggs. 



(&) Asterina gibbosa. Here also the eggs and the spermatozoa are formed in the 

 same genital organs, again not being simultaneously produced. The young animals 

 are males, the adults females. 



(f) Amphiura squamata. The simple pear-shaped gonads are very few in number. 

 On the average, the adradial and the abradial walls of a bursa have only one gonad 

 each sessile on it. The adradial gonads are testes, the abradial ovaries. Only a few 

 eggs in the ovary and a small number of spermatozoa in the testes ripen at one time. 

 These two kinds of sexual products here also, as it appears, do not ripen simul- 

 taneously in one and the same animal. The eggs are developed in the bursse. 



I. Care of the Brood and Sexual Dimorphism. 



Little by little, somewhat numerous cases of care of the brood have become known 

 among the Holothurioidea, Echinoidea, Asteroidea, and Ophiuroidea. These are not 

 infrequently connected with a more or less pronounced sexual dimorphism, the 

 adaptations for care of the brood being found only in the female. 



The eggs of an Echinoderm in which the brood is cared for are, as far as investiga- 

 tion on this subject has gone, distinguished by remarkable size, and by a rich pro- 

 vision of nutritive yolk, from those which are ejected into the water, and are destined 

 to develop into free-swimming larvae. 



The following review makes no claim to be exhaustive. 



(a) Holothurioidea. In /W/>x >///'/ /;;// (cf. Fig. 228, p. 287) large plates are 

 found on the back of the female, raised from the dorsal integument by means of 

 stalks. Between the stalks a brood chamber, roofed over by the contiguous plates, 

 arises ; in this the fertilised eggs which emerge through the dorsal genital aperture 

 pass through their development. 



In Cucumaria crocea, the developing eggs are retained in a dorsal trough, which 

 arises by the swelling up and bulging outward of the body wall in the two dorsal 

 radii. 



Another kind of care of the brood is found in Cucumaria hcvigata and C. minuto. 

 Two sacs here project from the body wall into the body cavity ; these are brood 

 pouches, which shelter the developing brood. The sacs are probably mere invagina- 

 tions of the body wall ; their outer apertures, however, have been discovered only in 

 0. minuta. The two sacs belong to the two ventral interradial areas ; in C. Iccvigata 

 they lie near the middle of the body, in C. minuta anteriorly. 



In Phyllophorus urna and Chirodota rotifcra, the body cavity serves as a brood 

 chamber. It is, however, unknown how the fertilised eggs pass in and the young 

 Holothurioidea out of it. 



In other Echinoderms, as might be anticipated, we find the spines occasionally 

 acting as protections for the brood. 



(6) Echinoidea. In a few Cidaroida (e.g. G. canaliculata, C. nutrix, C. mem- 

 branipora) the eggs are retained on the apical area of the test, and here develop, 

 protected by the spines, which bend together over them. The same is the case in 

 many Spatangoida, but the members of this order have become still more specialised 

 for this function. In certain forms either some or all of the petaloids (cf. p. 347) 

 sink in deeply, and thus give rise to brood chambers (marsupia) into which the eggs 

 pass from the genital aperture. The brood developing in such a marsupiurn is pro- 

 tected by the bending together of the larger spines which border it. In the Schizastcr 

 figured on p. 294, the anterior unpaired petaloid ; in Hemiaster cavcrnosus, in which 

 this arrangement is best known, the paired petaloids are the most deeply sunk. As 



