MUSEUM OF COMPARATIVE ZOOLOGY. 119 
great confidence in some of the speculations which have been indulged in 
until we know whether the Enteroccelen, and the Hydroccelen or water- 
tubes, originate in Ophiurans as paired structures or not, and whether the 
right Enteroccel is the same as the right ‘Lateral Scheibe” of Metsch- 
nikoff or not. The question also of the connection of the water “ tubes ” 
with the oral plate or madreporite is also imperfectly answered. It may 
be supposed that the Hydroccelen form from the Enteroccelen, which are 
themselves diverticula of the first invagination, and which persist in the 
form of the ‘ Lateral Scheiben.” 
Provisional Spines of the Pluteus. — Max Schultze * first made the in- 
teresting discovery that the embryo of the viviparous Amphiura syuamata 
is furnished with a provisional calcareous skeleton which is comparable 
with the spines of the arms of a pluteus. This important discovery has 
been verified by several observers. It presents us with a most interesting 
case of the retention of structures useful to free larva in an embryo where 
they can have no use, or if they have any use it must be a somewhat dif- 
ferent function from that which they have in the free pluteus. 
The provisional spines of the pluteus have a maximum development in 
the bisymmetrical larva (Fig. 8), but are not wanting in the younger 
stages of the pentagonal embryo, where they are very much reduced in 
size. The number, mode of origin, and position of the provisional spines 
of the pluteus seem to differ in different specimens. They are not always 
double or bisymmetrically arranged in reference to a plane passing through 
the mouth of the symmetrical larva and the umbilical connection with 
the parent. In the majority of instances the spine on the side opposite 
that in which lies the “left water-tube ” is well developed, while that on 
the same side as the left water-tube is stunted (see Figs. 6, 7). 
The provisional spines generally originate near the anal pole, but are 
found in some larve in the vicinity of the stomach and on one side of the 
body (Fig. 4, ps). In older stages of the bilateral larva the provisional 
spines form by reticulation a calcareous network (Fig. 10) similar to 
what we find in the anal lobe of Echinarachnius. This reticulation is 
hardly distinguishable from the permanent radial plates which form in 
the same position on the larva. The provisional calcareous rods are ulti- 
mately absorbed in the developing embryo, but do not wholly disappear 
until after the young Amphiura has passed into the pentagonal form (Fig. 
13). When last observed they were noticed just under the region of the 
* Op. cit., pp. 44, 45; Pl. I. Figs. 2, 3, 4,5 a. Krohn was not able to find a trace 
of the pluteus (op. cit., p. 340). 
. 
