68 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the primitive form, bermapliroditism or unisexuality. Among the 

 Annelida, the lowest forms are mostly unisexual ; only in the highly 

 modified forms, e.g. Oligoch.'cta and Myzostomida, is hermaphroditism 

 found. Another important fact is that " most hermaphrodites have 

 very complicated sexual ducts, &c.," whi"h are clearly adaptations 

 to bring about cross-fertilization ; such ducts could only arise in some- 

 what highly developed forms, that is to say, could only be converted 

 to secondary sexual uses in cases in which they were really present 

 beforehand, and hence only in somewhat highly developed forms. 

 This again i)oiuts to hermaphroditism not being the primitive condition. 

 In the genus Myzostoma the change from unisexuality to hermaphrodi- 

 tism has probably been brought about by the parasitic habit. The 

 chance of the continuance of the species would be largely increased 

 by the development in the female of testes in addition to the small 

 males, since the possibility of two individuals of opposite sexes coming 

 together on a single Comatula is clearly in many cases very small. 

 Finally, as Weismanu has suggested in the case of Daphnia, the males 

 might become extinct by a tendency to a periodic appearance ; the 

 periodicity being lengthened it would in time become infinite, and the 

 males cease to be developed altogether. 



'Challenger' Myzostomida.* — Dr. L. v. Graffs report on the 

 ' Challenger ' Myzostomida may be regarded as in some sense a 

 continuation of his monograph on this interesting and little-known 

 group. Of the sixty-eight species enumerated, fifty-two are new. 



These Myzostomes are small disk-shaped animals, living attached 

 to Crinoids, about whose aflfinities there has been up to the present a 

 good deal of doubt, some i)lacing them among the ^Vorms near 

 Tomopteris ; others, as von Graff, among the Ai'achnids, near the 

 Tardigrades ; and the discovery of a new form among the ' Challenger ' 

 collection seems to confirm the correctness of this latter view. The 

 author's class of Stelechojioda embraces the Tardigrades, Lingua- 

 tulids, and Myzostomes, thus constituting a group of very lowly 

 organized Arthropods. The report shows that the Myzostomida do 

 not form so imiform a group, either as to their habits or structure, as 

 was formerly thought. It is prefaced by a very neat though brief 

 account of the general structure of Myzostoma as far as it is known, 

 with a graphic coloured diagrammatic representation and most 

 minute details as to the general morphology, from which we condense 

 the following important statements : — 



While all the hitherto known forms were characterized by the 

 peculiar radial arrangement of the organs of the body, several species 

 are here described in which this arrangement is entirely lost ; in 

 some (M. folium) the body is greatly lengthened and the parapodia 

 and suckers are found arranged in two parallel lines, while in a 

 new genus {Stelecliopus) not only has the external radial symmetry 

 disappeared, but the muscular septa and parapodial muscles are not 

 even convergent ; hence, if, as the author believed long ago, the 



* Eeport of the Voyage of H.M.S. ' Challenger '—Zoology, x. (1884) 82 pp. 

 (16 pis.). See Nature, xxxi. (1884) pp. 165-G. 



