THE ECHINUS, OR SEA-URCHIN. 
367 
The whole structure will at first naturally he thought to be an 
external skeleton, like the true shell of a mussel,* or the hard 
investment of a lobster.| It is, however, nothing of the kind, 
hut is a truly internal skeleton, inasmuch as the soft substance 
of the animal (the perisoma) coats it externally as well as 
internally. 
The numerous, distinct, calcified plates which form the shell 
are arrayed in a very definite order. Of the two poles the 
inferior one is termed the 44 oral pole,” as it is there that the 
mouth is to be found. The superior one is called the 46 apical 
pole,” and this is the situation of the anus. Around the anus 
are placed some small and more or less irregular plates, termed 
44 anal plates.” External to and immediately below the anal 
plates are ten large ones, each perforated, and forming together 
a single circular row. These ten plates are the 44 ocular ” and 
44 genital ” plates, and they alternate with each other so that 
each ocular plate is between two genital plates, and each genital 
plate is between two ocular ones. The perforation in each 
genital plate is the external opening of a genital duct. The 
perforation in each ocular plate is for an eye-spot. Below this 
circle of ten plates is the great bulk of the shell, which is called 
the 44 corona ,” and consists of five vertical tracts beset with 
perforations and termed the ambulacra . Alternating with these 
are five other tracts of greater width, destitute of pores and 
termed the interambulacra . Each ambulacrum and each inter- 
ambulacrum is narrower at its apical than at its oral end, and 
consists of two vertical series of plates. Each of these plates 
is pentagonal, while two angles of each pentagon are right 
angles. A zigzag suture joins together the two vertical series 
of ambulacral plates in each ambulacrum, and a similar suture 
unites together the two vertical series of interambulacral plates 
in each inter ambulacrum. On the other hand, a straight 
suture unites the flat margins of each series of ambulacral 
plates with the flat margins of the adjacent series of inter- 
ambulacral plates. The plates which form the ambulacrum 
are seen, when closely examined, to consist each again of three 
pieces (pore-plates), which form between them the six pores, 
or foramina, of each ambulacral plate. 
The interambulacral plates, on the other hand, are single, 
and without perforations. 
There are never more than two vertical series of plates in 
each ambulacrum and in each interambulacrum. Therefore 
there are five pairs of rows of ambulacral plates and five inter- 
spaces occupied by five pairs of rows of interambulacral plates. 
* See the article on 11 The Anatomy of the River-Mussel,” by Mr. J. C. 
Galton, Popular Science Review for July 1870. 
t See the article on the Lobster in the same work for October 183S. 
