462 SUMMARY OF CURRENT RESEARCHES RELATING TO 



parasites ; from the fact that in many cases the diiferent sexes were 

 found in diiferent individuals of Amphiura, that in the plasmodium 

 sacs no zoosperms were detected, and that the adults are able to live 

 but a short time in sea-water, the author thinks it likely that fecunda- 

 tion is effected outside the body of the host, and a short time after 

 the parents leave it. 



If we sum up the characters of the Orthonectida we find them to 

 be forms which generally exhibit a radiate plan of structure, have a 

 ciliated and segmented dermal layer, well-developed generative organs 

 with well-marked dimorphism of the sexes. They can only be placed 

 with the Vermes on the supposition that that group does not represent 

 a definite phylum. Metschnikoff thinks it probable that they are 

 degenerated forms, and that their apparent simplicity of structure is 

 a secondary result ; this of course would make the question of their 

 true relationship very difficult ; they do not seem to have any close 

 relation to the Dicyemida ; the author is of opinion that Giard's idea 

 of their affinity to the Rotatoria is not to be disregarded, but he him- 

 self would suggest the Turbellarian Dhmphilus as their closest ally, 

 for this form has a superficial ciliary segmentation and well-marked 

 sexual dimorphism. The minute males are developed from special 

 ova, which are smaller than those for the females, and within them 

 the only evident organ is a spacious testicular sac. The topographical 

 relations of their jjarts must not be regarded as definitely indicating 

 any homology of germinal layers, and this remark may be extended 

 to the Dicyemida. The theory of Eabl, which teaches that the direc- 

 tion movements of an animal in a confined space are to be associated 

 with a radiate type, is not supported by what we know of these 

 creatures ; the Orthonectida — as indeed their name implies, and for 

 this reason was given them by Giard — move in a linear direction, and 

 yet exhibit a radial structure. 



Systematic Position of Balanoglossus.* — Elias Metschnikoff 

 returns to the view which he put forward twelve years ago, with 

 regard to the close affinity between this curious " worm " and the 

 Echinodermata. Such points as the two hinder circlets of cilia and 

 the terminal anus are seen in some Echinoderms, and the latter is 

 always found in the youngest stages of all typical Echinoderm larvae. 

 More important characters in common are found in the longitudinal 

 band of cilia, the water-vesicle opening by the dorsal pore and the 

 peritoneal sacs ; in fine, the differences between Bipinnaria and a 

 so-called vermiform larva of an Asteroid are more considerable than 

 those between Auricularia and the larval Balanoglossus, which is 

 known as Tornaria. The differences insisted on by A. Agassiz 

 depended partly on his view of the characters of the water-vascular 

 system, which the observations of Goette have shown to be erro- 

 neous. 



Similarities do not, however, end here, for they extend to the 

 details of histological structure. In both there are epidermal thick- 

 enings for the ciliary ridges, and the same kind of wandering 



* Zool. Anzeig., iv. (1881) pp. 139-43, 153-7. 



