THE ORTHONECTIDA 157 



to numerous individuals which in some Malacotyla as in Orthonectida 

 are destined to become sexually mature. Caullery and Mesnil consider 

 that the plasmodium of the Orthonectida corresponds to the axial cell 

 of the Dicyemida; the one as the other giving origin by endogenous 

 germ-formation to sexual ciliated forms. According to Wheeler, the same 

 axial cell of a Dicyemid produces successively females and males. There is 

 therefore close agreement with the facts observed as to the "plasmodium" 

 of the Orthonectida. Wheeler, however, suggests, but has not actually 

 observed, that the males (so-called infusoriform embryos) of Dicyemids 

 proceed from fertilised germ-cells only (see note, p. 152). E. R. L.] 



The Orthonectida and Dicyemida have been grouped together to 

 form a grade Mesozoa by v. Beneden a grade intermediate between the 

 Protozoa and Metazoa, and characterised as containing multicellular 

 organisms consisting of ectoderm and endoderm, the latter not being in 

 the form of a layer surrounding an enteric cavity. 



But Giard, Whitman, and others have shown that the groups do not 

 require the formation of this new grade to contain them ; they are in 

 reality Metazoa, for there can be no doubt but that the " intermediate 

 cells " of Orthonectida and, at any rate, the " primary germ cells " of 

 Dicyemida are mesodermic ; possibly the axial cell of the latter 

 represents an endoderm, reduced by parasitism to its last remnant a 

 single cell. In the Orthonectida, since the central cell of the embryo 

 gives rise to genital cells, it may be regarded as mesoderm, so that in 

 this group all trace of endoderm has disappeared. Nor is this fact 

 unique amongst parasites, for both in Cestoidea and in some Nematoidea 

 the gut is entirely absent. 



The simplicity of these organisms is not primitive, but secondary ; 

 they are degenerate Platyhelminths, but whether descended from 

 Turbellaria or from Trematoda is uncertain ; the resemblance to the 

 ciliated embryos of the latter class is more apparent than real, for they 

 are really much simpler than these. 



Owing to the gap between these two groups and the rest of the 

 Platyhelminths, Pagenstecher suggested the term " Mionelminthes " in 

 place of " Mesozoa." * 



APPENDIX II. 



Trichoplax adhaerens, F. E. Schulze, 2 occurs in the marine aquarium 

 at Graz, and is known nowhere else. It is a small, circular disc, moving 

 partly by means of cilia, partly by thrusting out simple pseudopodia-like 

 processes. The organism consists of an outer layer of ciliated cells, but 

 those on the lower surface, by which the animal creeps over the glass, 

 etc., are columnar ; those of the upper surface are flat. The central mass 

 of the body is composed of spindle-shaped and slightly branched cells, 

 leaving spaces filled with fluid between them. Nothing is known of 

 the process of reproduction beyond the fact that in autumn Schulze 

 observed (1) that the individuals were drawn out into long threads, and 



1 Bronn's Thierreichs, Wiirmer. 2 Zool. Anzeig. vi. 1883, p. 92. 



