272 COMPARATIVE ZOOLOGY. 
chiates, which breathe by the skin. The majority are 
fresh-water, as the Leech (Sanguisuga) and Earth-worm 
(Lumbricus). 38. The Branchiates, whose organs of res- 
piration are tuft-like gills on the back or head. Such are 
the sea-worms Avenicola, Nereis, and Serpula. 
The Jeotifers, or “ Wheel - animalcules,” are minute 
aquatic Articulates, whose exact position is doubtful, hav- 
ing some of the features of both Worms and Crustaceans. 
They resemble Infusoria externally, but have a complete 
alimentary canal and well-developed nervous system. At 
the head is a disk furnished with cilia for locomotion and 
prehension, and at the other end is a pair of “toes,” which 
act like forceps. None are over 3'5 of an inch long. 
Cuass I].—cCrustacea. 
This class includes all Articulates having jointed legs 
and gills.’ Among them are the largest, strongest, and 
most voracious of the subkingdom, armed with powerful 
claws and a hard cuirass bristling with spines. Although 
constructed on a common type, Crustaceans exhibit a 
wonderful diversity of external form: contrast, for exam- 
ple, a Barnacle and a Crab. We will select the Lobster 
as illustrative of the entire group. 
Every Crustacean consists of twenty-one segments, of 
which seven belong to the head, seven to the thorax, and 
seven to the abdomen. In the Lobster, however, as in 
all the higher forms, the joints of the head and thorax 
are welded together into a single crust, called the cephalo- 
thorax. On the front of this shield is a pointed process, 
or rostrum, and attached to the last joint of the abdo- 
men (the so-called “ tail”) is the sole representative of a 
tail—the telson. This skeleton is a mixture of chitine 
and calcareous matter, and corresponds to our epiderm- 
ister 
On the under-side of the body we find numerous ap- 
