98 HENRY BURY. 
It is not my purpose in this paper to discuss how or why the 
hydroccel came to encircle the cesophagus ; our present evidence 
does not seem to me to be sufficient to allow of even a plausible 
guess; but that it did assume this position before the general 
bilateral symmetry was lost, there is a good deal of evidence to 
show. In the first place, it actually does assume this position 
in Ophiurids ; and secondly, in many larve its plane forms a 
marked angle with that of the longitudinal mesentery—an 
arrangement easily derivable from that of Ophiurids. This is 
seen in Asterina (15, p. 156, fig. 3, taking the “antiambulacralen 
Armanlagen” as parallel with the mesentery), and in Crinoids 
(4, fig. 59). As we shall see in the next stage, this condition 
may be considered as a derivative from that seen in Ophiurids, 
approximating to a later arrangement; while it is not easy to 
understand, if this or the later arrangement (with the planes 
of hydroceel and mesentery parallel) is primitive, how the 
Ophiurid position was ever arrived tt. In Holothurians, on 
which Semon’s Pentactza is founded, this point is not so 
clearly defined as he supposed, as my description of Synapta 
will show; but as there is certainly nothing in this form 
opposed to the view that the Ophiurid arrangement is primitive, 
this need not detain us now; while the fact that no trace of this 
arrangement is seen in Bipinnaria asterigera or in Echinid 
Plutei is easily explained, when we consider that in these 
larve the hydroceel ring is arranged in relation to the secondary, 
not the primary, cesophagus. But all this will be clearer when 
we have considered the next stage in ancestral history. The 
same is true of several details in the positions assigned 
to the water-tube, &c., but the situation of the water-pore is 
clearly in accordance with what we find in most larve, while 
the adradial position of the opening of the water-tube into the 
water-vascular ring has been frequently commented on in the 
foregoing pages. It is this position of the water-tube, together 
with the probability of a symmetrical arrangement, which has 
led me to assign a dorsal position to one interradius ; for though 
it is tempting to assume a mechanical function for the posterior 
unpaired tentacle, and perhaps for the number (five) of the 
