MONOGRAPH OF THE EXISTING CEINOIDS. 653 



That these forms are descended from androgynous species, in which the organs 

 of one sex have gradually become abortive, is shown by those types in the females 

 of which there are rudiments of testes but no male generative aperture. These 

 difficious species are also distinguished by the marginal position of the sexual 

 apertures, both male and female, and by the form of the testes in the males. In 

 a few, which resemble the free-living forms in the possession of 20 long cirri, 

 the testes have the typical ramified form. In all the others they are compact, 

 rounded organs occupying definite areas in the lateral portions of the body. 



In those forms in which the individuals inhabiting one cyst are not different 

 in appearance the sexual organs have a different structure. Each individual in 

 this case is androgynous but differs from the free-living androgynous species in 

 that the testis is developed only on one side of the body, and there is but one 

 male genital aperture. In one species there are small remnants of the other testis 

 but no second male aperture. The testis also, as in the dioecious forms, is a small 

 compact gland. 



The male and female being found associated in a common cyst and increasing 

 in size with the growth of the cyst shows that they perforate the brachials or 

 pinnules of their host together. The growth of the cyst is caused by the presence 

 of the parasite. The female deposits her eggs within the cyst, and the young 

 embryos, after they have abandoned the cyst and lost their ciliated coat, associate 

 in pairs and bore their way through the brachials. 



In both sexes the sexual development begins with the appearance of testes, 

 but in the female the testes degenerate and disappear entirely, or leave but a 

 minute rudiment, when the ovaries make their appearance. 



Wheeler takes exception to von Graff's supposition that the young myzo- 

 stomes associate in pairs and together take part in forming a cyst. From his 

 studies of Mysostomum parasitieum and M. pulvinar, both of which show a dis- 

 tinct tendency to occur in pairs, each consisting of a senior and a junior individual, 

 he is led to believe that, in the case of the cysticolous species, the cyst must be 

 formed by a single individual, and that later a young myzostome. when it abandons 

 its pelagic trochophore stage, must enter through the aperture of the eyst and 

 settle down to a quiet life with the senior individual, and that the latter probably 

 dies at the end of its female stage and, undergoing decomposition, may perhaps 

 serve as food for its still vigorous junior partner, which in turn may become the 

 senior partner of another young myzostome. According to this view, all the 

 cysticolous myzostomes of a given species would not be cyst producing, but only 

 those which, instead of entering the orifice of a preformed cyst in their juvenile 

 stage, happen to settle between the brachials or contrive to work their way into 

 the calcareous skeleton of the crinoid. 



In species like M. cirri.ferum and M. parasitieum Wheeler recognizes during 

 their development the four following sexual phases : 



1. A phase of sexual neutrality, or indifference. 



2. A protandric phase, extending from the appearance of the first ripe sper- 

 matozoa to the appearance of the first ripe ova (male). 



