THE ORTHONECTIDA. 
251 
on the other side of the body, and the clear interspaces 
take the form of lozenges, just as one sees sometimes in 
looking through certain kinds of open basket-work. Finally, 
it will be seen below, that I have also found these elements 
in Intoshia, which Metschinkoff considers as the female form 
of Rhopalura, and where, consequently, there would be no 
spermatozoa. 
Whatever may be the origin of these muscular elements, 
they unite, as I have pointed out above, into a sort of fascicle 
at the terminal part of the adult animals, and by their 
contraction give rise to those brusque movements, those 
strokes of the club-like tail, to which the name Rhopalura 
has reference. 
If the interpretation above given is admitted, there 
would not be in the Orthonectida any true middle layer, but 
only a splanchno-pleural pseudo-mesoderm, comparable to 
the somato-pleural pseudo-mesoderm of the Hydra. 
I give to the totality of these elements the name o 
pseudo-mesoderm, because it appears to me desirable to 
reserve the name of mesoderm, properly so called, for other 
structures which do not exist in the Orthonectida, and the 
homology of which in the various groups of Metazoa is very 
difficult to establish. 
I distinguish : — (1) a solid mesoderm, formed very early at 
the expense of the endodermic cells of the embryo (rudi- 
ment of the notochord of the Tunicates and of the Vertebrates, 
skeletogenous cells of the embryo of Echinoderms, meso- 
dermal cells derived from the four first spheres of the endo- 
derm of Planarians, of Bonellia, according to the researches 
of P. Hallez and of Spengel, &c.). (2) K cavitary mesoderm 
formed by the diverticula of the endoderm (enterocoels), and 
appearing generally at a later epoch (aquiferous system of 
Echinoderms, enterocoel of the Tunicates, of the Brachio- 
pods, of Sagitta, of Amphioxus, &c.). The solid mesoderm 
gives rise chiefly to the muscular system ; the cavitary meso- 
derm forms principally the vascular organs. 
The physiological role of a histological element has, be it 
noted, only a secondary importance for the determination of 
phylogenetic homologies. A muscular element, for example, 
will always be formed at that point where it is needed, 
sometimes in a rudiment having an endodermal origin, 
sometimes at the expense of ectodermic elements (Nemer- 
teans). It may even be formed by a mere portion of a cell 
(plastidule), as we find in the Infusoria, in the Coelentera, 
and ill the Orthonectida. 
The interior of the endodermic sac is filled, in certain 
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