THE METAMORPHOSIS OF ECHINODERMS. 89 
morphic changes must be briefly reviewed before my own 
opinions are set forth. 
Semon (82) assumes a bilaterally symmetrical ancestor, 
which he calls “ Pentactea.” In it the hydrocel, which formed 
a ring round the mouth, had five tentacles, and was connected 
by the water-tube with the water-pore in the dorsal interradius. 
A dorsal mesentery, embracing the water-tube, ran back from 
near the mouth to the posterior end in the middle line; the 
mouth and anus were ventral, and the animal was fixed at a 
point somewhere on the dorsal surface. The representative 
of this ancestor he professes to find in the young stages of all 
Echinoderms (“ Pentactula”’ stage)—regarding the mesentery 
of the water-tube as identical with the longitudinal mesentery 
of the larva. He admits subsequent changes of position of mouth 
and anus, but onlyin Crinoids does he recognise the parallelism of 
the mesentery (or a part of it only, as he thinks) to the plane 
of the hydrocel, and accounts for it briefly as due to a 
secondary “ Drehung des Darmes.” 
How utterly he has misunderstood the position of the larval 
mesentery in Asterids and Kchinids will be evident without 
further comment to any one who will compare his figures 
(32, pl. vi, figs. 4 and 5) with the descriptions given in the 
foregoing pages of this paper; while Ludwig’s observations on 
Cucumaria (16 and 17) and my own re-examination of Synapta 
(on which Semon’s views are founded) suggest that the 
symmetry of adult Holothurians has not been quite so simply 
derived from that of the larva as Semon would have us believe. 
In fact, without entering further upon the details of his theory, 
I think we may say that his fundamental assumption of the 
retention of bilateral symmetry by the complete radiate form is 
opposed to embryological evidence. To the further assumption 
of the fixation of the common ancestor I shall return later. 
Biitschli’s theory (6) is extremely ingenious and very 
carefully reasoned ; but, while resting on no personal observa- 
tion, is a little too prone to ignore, or set aside on purely 
theoretical grounds, those statements of other observers which 
do not conveniently fit into it. It is impossible here to review 
