I 94 ] 
has thin spines nearly a foot in Itngth. Very conspiciiaus arc 
its five eyes of brilliant blue colour which are arranged around 
the veni. It is apparently very well able to see with them, as it 
always moves its spines in the direction from which one 
approaches it. Rarer locally is Phyllacanthus annuiifera, obtain- 
able by dredging in a few fathoms of water Its spines are 
thick, about 2 inches in length and are beset with whorls of 
spikes. The Slate-pencil Urchin (Heterocenfrotits mammillatiL'i} 
has exceedingly thick spines. A specirnen from Cocos Keeling 
L, presented by Mr, Ladds* is exhibited, Asthen&soma heteractis 
is locally of rare occurrence, but may be dredged in Keppel 
Harbour, It is of a somewhat flattened shape. Nobody who 
ever took it personally out of the dredge is likely to forget the 
occasion. It causes the most painful burning sensation on 
one's hands, due to poison glands at the bases of the spines. 
Some Sea-Urchins, like Strongyhcentrottts and Echinometra, live 
on rocky coasts, exposed to the full force of the waves. They 
are found in holes from which they can be dislodged only with 
the greatest difficulty, their size just fitting that of the hole. It 
is a matter of dispute whether they make these holes, perhaps 
by the continuous movement of their spines, or whether they 
merely wander into a suitable crevice and reniain there until by 
their increase in size they just fill up the place. Such 
Sea-Urchins occur on Christmas Island. 
The Cake Urchins, also called Shield Urchins or Sand 
Dollars, are disk-shaped and quite flat, barely of the thickness 
of a cent piece. Various species of Arachnotdes, PeroncUa and 
Laga?ium are fairly common on sandy shores, like Tanjong 
Katong. Lobophora is similar, but has two narrow slits in its 
shell. These Urchins are more or less bilaterally symmetrical. 
The ambulacral pores are arranged in leaf-like patterns, and 
the vent has in most cases shifted to the under surface. The 
spines are extremely small. 
The Heart Urchins, locally represented by Bn'ssus carina- 
tu,% are heart-shaped. Whilst resembling the Cake Urchins by 
their bilateral symmetry and the leaf-like arrangement of the 
ambulacral pores, they dift'er from them by being much thicker, 
by having much longer spines and by the absence of Aristotle*s 
lantern. 
SEA-CUCUMBERS (Holothuroidea). 
We call the principal axis of the body of an Echinoderra 
a line running from mouth to vent. In a Starfish this axis is 
