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CRUSTACEANS. 
exceeding an inch in length. As in Cystosoma, the second pair of antennae are 
obsolete; the head is large, with the eyes placed upon its summit. There are 
seven pairs of large thoracic appendages, the third 
from the end forming a large and strong pincer. 
The 
most abundant in the tropics. Like many pelagic 
animals, they are translucent, and mostly live in the 
mantle - cavity of the ascidians Pyrosoma and 
Doliolum, where the eggs are laid, and the young 
hatched. 
To a certain extent connecting the Malacostraca 
with the Entomostraca is a group of Crustaceans 
known as the Leptostraca, and containing the three recent genera Nebalia, 
Nebaliopsis, and Paranebalia, and a number of fossil forms. The affinities of 
the group seem to lie with the Phyllopods on the one hand, and the Schizopods 
on the other. The body is laterally compressed, and the whole of the cephalothorax 
and the first four segments of the abdomen enveloped in a carapace, which springs 
from the head, and is formed of two movable valves, closed by a muscle. Although 
the eight thoracic segments are overlapped by the carapace, they are distinct and 
movable. The abdomen consists of eight movable segments, or two in excess of the 
normal number; but there are only nineteen pairs of appendages. The head bears 
a small, movable rostrum, and a pair of stalked eyes. The two pairs of antennae 
are well developed, and there are three pairs of jaws. The appendages of the thorax 
are foliaceous. The members of this group are marine, and widely distributed, 
being found in cold and warm latitudes. The female carries the eggs attached to 
her thoracic feet. 
Subclass Entomostraca. 
The Crustaceans of this division are small, and vary much more than the 
Malacostraca, from which they differ in the following features. The number of 
body-segments is not constant, but either greater or less than nineteen, and, as a 
rule, there are no appendages to the abdomen. In the majority of cases the young- 
are hatched as a Nauplius. 
The Barnacles, —Order Cirripedia. 
The adult members of this group are so unlike typical Crustaceans that it can 
hardly be a reproach to the older naturalists that they failed to discover their 
affinity. Two well-known members of the order are the barnacles so frequently 
attached to the bottoms of ships or floating timber, and the acorn-barnacles 
covering the rocks on the coast. The barnacle ( Lepas ) consists of a tough longer 
or shorter stalk, one end of which adheres tightly by means of a cement to the 
timber or ship, while to the other is attached an oval compressed body encased in 
pieces of shell, through two of the valves of which can be protruded six pairs of 
slender, bristly, two-branched, filamentous limbs. These limbs, being the appendages 
of the thorax, keep up a constant sweeping motion, whereby particles of food are 
species are widely distributed, although 
