GENERAL INTRODUCTION
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structure of the animal is totally unlike that of the bivalve
'
molluscs.
The TU_NICATA are exclusively marine and—,comprise many animals of very different appearance… There
are the common sea squirts of our shores and shallow’ seas,‘
.
either solitary creatures like little gelatinous .,orleathery
bags fastened on to rocks, or else great numbers of smaller
individuals embedded in a common gelatinousmass, solitary
and
colonial Tunicates respectively.
Each individualhas
two openings WhiCh, when theanimals are squeezed, squirt
out a stream of water, hence their commonname. — Other —
varieties of Tunicates, called Salps and Appendicularians,
form an important partof the drifting life of séassomewhat
.
warmer than our own, such as the Mediterranean;
They
are
transparent animals, which may be solitary but are '
often fastened together" in long chains.
—
-
.In their early stages the Tunicates re'semble little tadpoles
With a backbone whichlater disappears but the possession
of - which maymean that they are really
.
relations
of the VERTE3RATEs—distinguished from the
INVERTEBRATES,ÆVÏ1ÏCÏl we have hitherto_ been considering,
:
.
_
by the possession of a backbone-.
The simplest animals
to show certain Vertebrate characteristics are the Lancelets,
‘
.
like little white eels ivhich_burrow in the sand, while, rather
simpler than the true sh, are the CYCLOSTOM_ATA, of which
the lamprey is our chief example. *They. have
.
a
larger
number of gill openings than the true sh, have round
months with no true teeth_ancl have no paired ns on the
sides of the body.
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The sh or -Prsces are too well—known to need description.
They are divided into two principal groups, one, called the
Elasmobranchs, having a relatively soft, cartilaginous
.
skeleton and with the gill openings separate—to mention
_
two of their chief characteristics—and including the dog—sh,
Sharks and skatés, and the other, knoWn: as the Teleosts