THE HOMOLOGIES OF PEDICELLARLE. 399 
as it stood in the days of Valentin in spite of the many observa- 
tions made, and hints of their true nature thrown out by Miller, 
Troschel, Sars and A. Agassiz, which would have saved 
errier much useless speculation. 
No attempt has yet been made to ascertain the homol- 
ogies of these organs, and the present article is pig. 79, 
intended to give the results which have been 
reached by the writer since 1864, from the study 
of the embryology of starfishes and Echini. 
If we examine the common sea-urchin of the 
coast of New England, we shall find, scattered in 
between the spines over the whole surface of the 
shell, numerous pedicellarize (Figs. 78 and 79). 
They consist of a calcareous stem (Fig. 80) artic- O 
Fig. 78. 
Toa E a a 
- ° is surrounded by a muscular sheath expanding into a 
2 somewhat swollen portion with a thimble-shaped knob at the end. 
This knob, though it seems solid and compact at first sight, is in 
reality split into three wedges (Fig. 81a), which can rig. sia. 
be opened and shut at will. When open, these pedi- 
cellariæ may be compared to a three-pronged fork, 
Fig. so, except that the prongs are arranged concentri- 
cally instead of on one plane and when closed 
they fit into one another as neatly as the pieces 
of a puzzle. Fig. 81 represents the end view 
of one of these pedicellariz. 
If we watch a sea-urchin after he has been 
feeding, we shall learn at least one of the 
offices which this singular organ performs in 
the general economy of the animal. That part 
of the food which he ejects passes out of the 
anus, an opening on the summit of the body in the small 
area where the zones of which the shell is com- 
posed converge. The rejected particles, thrown 
out in the shape of pellets, are received on 
these little forks which close upon them like forceps, 
and they are passed from one to the other down the 
Side of the body till they are dropped off into 
Water. Nothing is more curious and entertaining than to watch 
the neatness and accuracy with which this process is performed. 
