ECHINODERM MORPHOLOGY. 
323 
histological structure and of their functions as afferent 
channels of the water-vascular system. But this similarity 
does not constitute an homology. The dorsal pore, which 
is developed so early in the other Echinoderm larvae as the 
foundation of the madreporic system, is absent from the 
corresponding stages of the Antedon larvae ;• and until the 
development of the primary water-pore and water-tube of 
the Crinoids are more fully known, I do not think that we 
can without qualification accept Ludwig^s view regarding 
their homology with the madreporic system of other Echino- 
derms. The following remarks, however, are based upon 
the assumption of the truth of that view. 
Ludwig regards the external opening (water-pore) of the 
madreporic canal (water-tube) as a fixed point. Whether it 
be actinal or whether it be abactinal does not matter. In 
the larvae of some Crinoids and in the adult Ophiurids it 
opens on one of the interradial actinal plates (orals) around 
the mouth. In the adult Urchins and Starfishes, however, 
it opens on one of the interradial abactinal plates around 
the anus (genitals). This is far from being the case in 
young forms of either class. Nevertheless, Ludwig thinks 
the relations of the parts in the adult sufficiently fixed to 
enable him to assert the undoubted homology of the orals of 
Crinoids and Ophiurids with the genitals of Urchins and 
Starfishes. He attempts to strengthen this remarkable 
position by other considerations to which I shall presently 
advert; and he points out how diametrically opposed it is 
to the view now generally held that the Basals of Crinoids 
represent the genitals of Urchins and Starfishes.^ He seems, 
however, not to attach much value to the evidence on which 
^ I have often wondered whether the “ pseudoproct ” of Wyv. Thomson, 
a depression at the hinder end of the very young Antedon larvse, can be 
regarded as representing an undeveloped dorsal pore. Its position at the 
end of the transverse Gastrula-axis is the same as that of the dorsal pore 
in other Echinoderm larvae, though I am bound to admit that it is not dorsal. 
But owing to the extreme elongation of the Gastrula along that axis it is 
very far from the water-vascular ring and not close to it as in other larvae. 
This might account for its want of further development. On the other hand, 
the primary water-pore of the Antedon larvm leads into the oral ccelom 
developed from the left peritoneal vesicle of the embryo. It is not likely, 
therefore, ever to have been a simple dorsal pore, or it would have opened 
into the aboral coelom developed on the dorsal side of the larva from the 
right ])eritoneal vesicle, su])])osing of course that it were not directly con- 
nected with the water-vascular ring by a water-tube, as in other Echino- 
derms. 
“ See “The Oral and Apical Systems of Echinoderms,” this Journal, 
New Scr., yols. xviii and xix ; and also the recently published text-books 
of Claus, Zittcl, and other authors. 
