MUSEUM OF COMPARATIVE ZOOLOGY. 119 



great confidence in some of the speculations which have been indulged in 

 until we know whether the Enterocoelen, and the Hydrocoelen or water- 

 tubes, originate in Ophiuraus as paired structures or not, and whether the 

 right Enterocoel is the same as the right "Lateral Scheibe" of Metscli- 

 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 Hydrocceleu form from the Enterocoeleu, which are 

 themselves diverticula of the first invagination, and which persist in the 

 form of the " Lateral Scheiben." 



Provisional Spines of the Pluteits. — Max Schultze * first made the in- 

 teresting discovery that the embryo of the viviparous Amphiura squamata 

 is furnished with a provisional calcareous skeleton which is comparable 

 Avith 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 diff"er in difi"erent 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 larvae 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 embiyo, 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 



* Oj\ ciL, pp. 44, 45 ; PI. I. Figs. 2, 3, 4, 5 a. Krolui was not able to find a trace 

 of the pluteus (op. cit., p. 340). 



