THE METAMORPHOSIS OF ECHINODERMS. 97 
and under it a “dorsal organ” extending probably forwards 
towards the water-vascular ring ; but though I have introduced 
these into fig. 45, I shall, for the sake of clearness, omit them 
almost entirely from my subsequent description, returning to 
them when I deal with the relation to Enteropneusta. 
The ancestral form just described is closely similar to Semon’s 
** Pentactza,” from which it differs in not being fixed, and in 
the body-cavities not extending forwards far enough to form a 
mesentery enclosing the water-tube. Still more closely does it 
resemble an Ophiurid Pluteus, just before metamorphosis, 
deprived of its arms and its pelagic habits; indeed, almost the 
only important points of difference lie in the facts that the 
Pluteus has a pentamerously-arranged skeleton (which I shall 
deal with immediately), but has not a very definite atrial 
cavity, which, however, is not essential to the ancestor. 
One of the first questions which meets us in trying to 
reconstruct the history of Echinoderms is—what organ, or 
group of organs, originated the pentamerous arrangement ¢ 
The Sarasins (28, p. 147) have given precedence on this point 
to the longitudinal nerves and muscles; but I think most 
embryologists will be inclined to follow more closely the 
teachings of ontogeny, which seem to point to either the 
hydroceel or the skeleton as the first to exhibit this symmetry. 
Now in spite of the markedly metameric arrangement of the 
skeleton in many larvee, there are very serious difficulties in 
the way of making it the starting-point of the pentamerous 
symmetry; indeed, all homologies of the skeleton between 
Pelmatozoa and Echinozoa have been of late strenuously denied. 
But without entering upon this very difficult question at 
present, let us examine the primitive nature of the five pouches 
of the hydroceel, and see to what results it will iead us; for, 
after all, such assumptions must be judged more by the results 
deducible from them than by the direct evidence in their 
favour—provided always that ontogeny establishes a fair 
prima facie case, which is not, I think, true of the Sarasins’ 
supposition. 
vou. 38, pART 1.—NEW SER. Ga 
