INTRODUCTION. 
XXV 
another, sucked in, as it were, through an imperceptible 
orifice. 
Acontia arc less universal than craspeda, for whereas 
the latter arc always present, so far as I know, in this 
order, the foiTner are found only in the Sagartiadce, and 
perhaps in the Bunodidce. In Sagartia helUs they spring 
from the mesenteries that carry the craspeda; generally 
two acontia from each mesentery, and most frequently 
in pairs. Their point of insertion may be anywhere in the 
length of the mesentery, great irregularity prevailing in 
this respect. 
Though at first it seems a solid cylinder, the acontium is 
really a flat narrow ribbon, with involute and approximate 
edges, which can at pleasure be brought into contact, 
and thus constitute a tube (Plate XI. fig. 4). Like the 
crasjjediim, of which it seems to be a form modified for 
a special use, its surface is richly ciliated ; and the ciliary 
currents not only hurl along whatever floating atoms chance 
to approach the surface, but cause the detached fragments 
themselves to wheel round and round, and to swim away 
through the water. Though there is not the slightest 
trace of flbrillm in the structure of the acontium, even under 
a power of 800 diameters, the clear sarcode, of which 
its basis is composed, is endowed with a very evident 
contractility. 
Under pressure, the edges of the flattened acontium 
appear to be thronged with clear viscous globules, over- 
lapping one another, and protruding ; indicating one or 
more layers of superficial cells, doubtless forming the 
peritoneal e 2 )ithelium. As the pressure is increased, these 
ooze out as long pear-shaped drops, and immediately 
assume a perfectly globular form, with a high refractive 
power. Below these is packed a dense crowd of cnidee, 
arranged transversely. 
The Cinclides. The emission of the acontia is provided 
for by the existence of special orifices, which I term 
Cinclides. The integument of the body, in the Sagartice, 
is perforated by minute foramina, having a resemblance in 
appearance to the spiracida of insects. They occur in the 
interseptal spaces, opening a communication between these 
and the external water. 
The appearance of the cinclides may be compared to 
that which would be presented by the lids of the human 
