6 
CEETACEOUS ECIIIXODEinrATA 
wliicli, near the apex, lies in a hole of the so-called ocular plate, at the superior end 
of each amhulacrum. Although no lens was observed in these fibrous globular 
teriniuations, they arc believed to represent eyes, and hence the name- ocular plates. 
Of the other senses, only touch is developed in the various soft appendages of the 
external, body particularly the pcdicclls. 
The sexes appear to he always distinct. Both, whether ovaidcs or testes, consist, 
as far as observed, of five principal portions ; in the hemispheroid forms their length 
varies. They are chiefly situated along the inter-ambulaci-a, forming greatly sub- 
divided and branching glands. Each terminates with a pore in tlic so-called genital 
plate, of which there arc in the regular forms of Echinoidca generally five, in the 
irregular group often four or three, Avhile in some of the palaeozoic Tesselati their 
number rises to fifteen. There is no external difference in the sexes, but internally 
the organs are of course very dilferent as regards the elements of secretion. The 
testes are usually white, but the ovaries yellowg brown, pink, or of a bright red 
colour. 
The development of the young begins with the ovum, and passes through a rather 
complicated metamorphosis. It has as yet been observed only in a few species of 
Schkaster, Spatangus, Toxopneiistes, etc. The furcation of the yolk in the egg 
begins a few hours after fructification has taken place. After the first day the egg- 
membrane breaks open, and the globular or oval embryo moves about Avith the 
support of its cilia, and shortly after the first external traces of the calcareous 
covering appear by the formation of three to five protuberances round the mouth, 
which becomes then distinctly traceable, Avhile the opposite end is somewhat pyra- 
midal. The anterior prolongations multiply somewhat differently in various genera, 
and increase in length. When after about a month this stage had been attained, 
both the pyramidal posterior and the anterior prolongations become graduallv 
absorbed, while all the internal organs are well indicated ; at the end of the posterior 
prolongation appears the rudiment of the madreporiform plate, towards Avhich the 
aquiferous system concentrates. After some time the larval pyramidal form with 
the calcareous cylinders gradually disappears, when pedicells and pedicellaritc have 
become developed on the surface of a small excentric protuberance which appears 
at the side of the mouth ; some cylindrical spines also become apparent, the last 
remains of the larva vanish, internally the calcareous plates are formed, and the 
young Echinoid gradually assumes the form and shape of the old. 
The Echinoids effect their locomotion and attachment to foreign bodies through 
the different kinds of pedicells, and also the spines, the latter serving besides as 
organs of defence. All are marine inhabitants, mostly in moderate depths. Some 
live on sandy or gravelly ground, others on coral reefs, hiding in crevices, or even 
excavating holes in solid rocks with the abrading poAA’cr of their spines. Though 
also found in northern and temperate seas, they are of course most numerous and 
most varied in tropical climates. In former periods they also appear to have been 
generally distributed, but their number Avas smaller. 
( 70 ) 
