326 
P. HERBERT CARPENTER. 
water-tube, which extends backwards from the water-vascular 
ring and not forwards as it should on Ludwig’s theory. 
Again, what is the position of the five water-pores of Tri- 
chaster elegans ? Not close round the mouth in the position 
of the absent mouth-shields, but between the two genital 
clefts of each interradius. The interradial plates sometimes 
developed here in other Ophiurids have been hitherto re- 
garded as representing the genitals of Starfishes and Ur- 
chins; and the presence of the water-pores in the same posi- 
tion in Trichaster elegans goes a long way towards strength- 
enins: that view, while at the same time it diminishes the 
probative value which Ludwig assigns to the perforation of 
the mouth-shields by the water-pores in other Ophiurids. 
Besides the perforation of one of the orals of the larval 
Crinoid by the water-pore, the only other arguments adduced 
by Ludwig as showing the homology of these orals {vice 
the basals, discarded) with the genitals of Urchins are the 
following: — (1) The anus is adoral from the basals of a 
Crinoid, but aboral from the genitals of a regular Urchin ; 
though it is aboral from the orals of a Crinoid. I would 
urge two considerations in reply. There are Crinoids in 
which the mouth is at the margin of the disc, close down 
to the abactinal skeleton, while the anus is central ; and I 
do not see why these and the irregular Urchins should be left 
out of consideration, especially when we remember that the 
mouth of the Pentacrinoid larva is excentric, as Ludwig 
himself has told us, and that the exocyclic Crinoids present 
other embryonic features. 
In the young Starfish the anus is primitively adoral from 
the genital plates, and only secondarily assumes the intra- 
genital position on which Ludwig bases his argument. 
Here, as in other cases, Ludwig’s homologies seem to me to 
rest too much on the variable secondary relations of organs 
as existing in the adult, and too little on the constant and 
primary relations of their rudiments in the larva. 
(2) Ludwig lays great stress on the homology of the oral 
side in all Echinoderms, as shown in the following interest- 
ing facts. When viewed from the oral side the gut always 
winds from mouth to anus in the direction taken by the 
hands of a watch, and the water-pore both of the Urchin and 
of the embryo Crinoid is in the same interradius as the fore- 
gut. He gives an excellent figure (Tab. XIII, fig. 7) of 
tl)e course of the gut in an Urchin as seen from the ventral 
side, and therefore (of course) represents the madreporite 
as seen from within in the N.w. corner of the figure. He 
gives a similar figure of the course of the gut in Antedon 
