THE ORTHONECTIDA. 
239 
IX. — General Reflections. 
At first sight it seems as though the discovery of the 
Orthonectida would bring a very solid support to the Planula 
theory of Ray Lankester, or, still more, to the Parenchy- 
mula theory of Metschnikoff, and I do not doubt that more 
than one zoologist will interpret, in that sense, the observa- 
tions I have recorded. 
1 persist, however, for my own part, in considering the 
invaginate Gastrula as the prototype of the Metazoa. 1 
base this opinion on the following arguments : 
1. The Orthonectida are parasitic animals, and we must 
take into account the retrogression which this kind of life 
may have brought about in their structure. An organisa- 
tion which we consider as one of primitive simplicity is very 
possibly simple only in consequence of degeneration, if we 
have to deal with a parasite, and especially an internal 
parasite. 
2 We have seen that the Planula is formed by epibole in 
Rhopalura ; in this case we have, then, at any rate, mo- 
mentarily, a real gastrula, which closes up and does not 
reopen, because the mode of life of the animal does not 
require the existence of a permanent digestive tube. 
3. The forms which present the embryonic phase termed 
parenchymula by El. Metschnikoff cannot be considered, as 
that naturalist would wish, as the lowest among the Sponges 
and Hydroids. 
The forms known as Halisarca are not low sponges, but 
sponges which have undergone a degeneration of their 
skeletal apparatus. From the point of view of general 
morphology I have shown that we may compare them to the 
Botrylli, and must assign to them a very high grade among 
the Fibrosa, analogous to that which the Botrylli occupy 
among the Synascidiae or the Leucones among the Calci- 
spongiae. 
The Siphonophora, we may remember, are very far from 
being Coelentera of inferior position, and it is by no means 
astonishing that they present a condensed embryogeny. 
The typical embryonic form is found among the Coralligena 
and certain Actiniae. 
The Echini and the Ophiurids with pelagic embryos have 
a small nutritive vitellus. They present a gastrula formed 
by invagination, and it is only when this gastrula is thus 
formed that the mesoderm takes origin, at first by partial 
delamination of the ectoderm, and later by lateral thicken- 
ings of the endoderm. 
