THYROSTRACA li 
tail-spine. Some authorities hold that this order should 
be removed from the crustacean class to that of the Arach- 
nida. The name of the third order, the Trilobita, refers to 
the circumstance that they usually have the body divided 
by two longitudinal dorsal grooves into three lobes. They 
were extremely abundant in bygone ages, and the natu- 
ralists of the Challenger were continually in hopes that 
they might obtain a living specimen or two from hitherto 
unexplored abysses of ocean. But extinction appears to 
have done its work with great thoroughness upon this 
order. 
The last of the sub-classes consists of the Cirripedia or 
curl-footed animals. The alternative name Thyrostraca, 
meaning ‘ valve-shells,’ has the merit of agreeing in ter- 
mination with the names of the other three sub-classes. 
But it must be admitted that if it is objectionable to call 
the whole group cirripedes when some have no cirri, it is 
equally inappropriate to call them all ‘ valve-shells’ when 
some have no valves. It is a triumph of the present cen- 
tury in minute investigation and comparative anatomy, 
that has withdrawn the Cirripedes from the zoophytes, 
worms, and molluscs, among which, at various times, the 
older naturalists placed them, and that has given them 
henceforth an undoubted position among the Crustacea. 
They may be divided into five orders, or the first two, 
the Pedunculata and the Operculata, may be grouped 
together as divisions of an order hitherto designated 
Thoracica, in which the part called the thorax is provided 
with cirri. ‘The Abdominalia have the cirri only on the 
so-called abdomen. ‘The Apoda are without cirri, being, 
as their name implies, footless. Lastly, the Rhizocephala 
are a parasitic set, which send rootlike filaments into the 
bodies of their hosts, 
