ON THE HISTOLOGY OF THE PEDICELLARLA. 385 
A, Aaassiz,* in the Revision of the Echini, gives a number of excellent 
figures of the calcareous parts of the pedicellaric of different kinds of sea 
urchins, besides summing up the facts known about the functions and develop- 
ment of these organs. The development is most satisfactorily to be studied in 
starfish, because in these animals the Pedicellariz are in many cases massed 
round the bases of the larger spines, and so it is possible to say from the first 
what will become a spine, and what a pedicellaria. This distinction cannot be 
made in Echini. Both spines and pedicellariz arise as a prolongation of the 
calcareous network at a very early stage; presently a constriction is formed at 
the base, and a tubercle on the surface of the test to which the spine is articu- 
lated. In the case of the pedicellariz the calcareous prolongation becomes 
bifid, but in the earliest stages the two organs are absolutely indistinguishable. 
Hence the homology of the two organs is clear ; and if anything further were 
needed to prove this homology, it is the presence in’ Porocidaris of pedicellarie, 
surrounded by a scrobicular area like that which surrounds the spines. 
In 1871 Mr Stewart t+ figured the pedicellariz of Cidaris, and in 1874 Sir 
Wrvitte THomson}{ figured those of Echinothuria, Porocidaris, and other 
genera. 
Up to the present time, therefore, as has been shown in the foregoing 
historical sketch, researches on the histology of pedicellarize of sea urchins have 
been chiefly directed towards their hard parts. Of all the authors mentioned 
VALENTIN§ has studied the soft parts most. The following are the results 
which he arrived at, stated as briefly as possible :— 
Pédicellaires gemmiformes.—The soft parts consist of a layer of epithelium, 
a pigmented layer, and a fibrous membrane. The muscles he could not clearly 
distinguish. 
P. tridactyles—The soft parts of stalk consist of a layer of epithelium, a 
pigment layer, and longitudinal and transverse (?) muscular fibres. 
P. ophiocéphales.—A “ring” or triangle of transverse muscular fibres. The 
stalk he describes as having the same structure as the last variety, but con- 
taining longitudinal canals which he regards as blood-vessels. 
Quite recently Mr SLapEn || has investigated the histology of the gemmiform 
pedicellarize by the help of modern methods, but we shall allude to his results 
farther on. 
Since the appearance of the abstract of this paper in the Comptes Rendus, 
* ALEXANDER Aaassiz, ‘“ Revision of the Echini,” Memoirs of Harvard Museum, vol. iii. 
+ “On the Minute Structure of Certain Hard Parts of the genus Cidaris,” Quart. Journ. Micro, 
Science, 1871, p. 51. 
t “On the Echinoidea of the ‘ Porcupine’ Deep-sea Dredging Expeditions,” Phil. Trans., 1874, 
p, (19. 
$ VALENTIN, loc. cit. 
|| W. P. Suapgn, Joe. cit. 
