2 A HISTORY OF RECENT CRUSTACEA 
wormlike genus called Balanoglossus; (4) the Vermes or 
Worms; (5) the Arthropoda; (6) the Mollusca, among which 
are the well-known oyster, snail, and cuttle-fish ; (7) the 
Molluscoidea, containing the mollusc-like lantern-shells, 
and the grouped animals of the Polyzoa, in some ef which 
the so-called ‘ bird’s-head’ organs amuse the observer; 
(8) the Tunicata, the tunic-clad or mantled animals, com- 
prising the Ascidians, whether tough-coated or gelatinous, 
and the Salpz which roam the sea in alternate genera- 
tions solitary or connected in a chain; (9) the Vertebrata, 
with the important classes of fish, amphibians, reptiles, 
birds, and mammals. 
It is with the central group of these nine that we are 
here concerned. So far as the name goes the Arthropoda 
are animals with jointed limbs. So far as the name goes, 
therefore, cats and dogs and vertebrates in general might 
belong to this division. But the name was given with 
reference not to the vertebrates, but to the vermes, for 
originally the worms and arthropods were included in a 
division called the Annulosa, animals of which the bodies 
contain several annuli, rings, metameres, somites, zonites, 
arthromeres, or segments, as they are variously called. 
These two sections of the Annulosa are now severed, and 
are distinguished by the circumstance that the one is, and 
the other is not, provided with jointed limbs. 
The Arthropoda are defined as animals which have 
bodies composed of variously shaped segments; which 
have jointed appendages attached to some at least of the 
segments; which have (in general) a brain united to a 
ventral nerve-cord, or ganglionic chain, and which exhibit 
bilateral symmetry. 
None of the other divisions will be found to possess all 
these characters combined. For example, in the verte- 
brata the nerve-cord is dorsal, in the mollusca the body is 
unsegmented, in the vermes there are no jointed appen- 
dages. Instances, it is true, are to be met with of arthro- 
pods which do not themselves answer the requirements of 
the definition, instances in which the body is unsymme- 
trical or unsegmented, and in which there are no articu- 
