268 ZVOLOGY. 
1s a general rule that in the Arthropods each segment bears 
but a single pair of appendages. The abdominal feet are 
called ‘‘swimmerets ;” they are narrow, slender, divided at 
the end into two or three lobes or portions, and are used for 
swimming, as well as in the female for carrying the eggs, 
The first pair are slender in the female (Fig. 221, B 9 ) and not 
divided, while in the male (Fig. 221, B24) they are much 
larger, and modified to serve as intromittent organs. The 
sixth segment (Fig. 221, G) bears broad paddle-like append- 
ages, while the seventh segment, forming the end of the 
body and called the “telson,” bears no appendages. It rep- 
resents mostly the tergum of the segment. Turning now to 
the cephalo-thorax, we see that there are two pairs of an- 
tenn, the smaller pair the most anterior ; a pair of mandi- 
bles with a palpus, situated ori each side of the mouth; 
two pairs of maxille or accessory jaws, which are flat, di- 
vided into lobes, and of unequal size ; three pairs of foot-jaws 
(maxillipedes), which differ from the maxille in having gills 
like those on the five following pairs of legs. There are thus 
thirteen pairs of cephalo-thoracic appendages, indicating that 
there are thirteen corresponding segments; these, with the 
seven abdominal segments, indicate that there are twenty 
segments in a typical Crustacean. By some authors the eyes 
are regarded as homologues of the appendages, but in early 
life they are seen to be developed on the second antennal seg- 
ment, as they are in the lower Crustacea. They are simply 
modified epithelial cells of the body-walls, as in the eyes of 
the lower invertebrates. ‘The ears are situated in the smaller 
antenne (Fig. 221, a’). In the second or larger antenne are 
situated the openings of the ducts (Fig. 221, h) leading from 
the ‘‘ green glands,” while the external openings of the ovi- 
ducts are situated, each on one of the third pair of thoracic 
feet. 
It is impossible, except by counting the appendages them- 
selves, to ascertain with certainty the number of segments 
in the cephalo-thorax, the dorsal portion of the segments be- 
ing more or less obsolete, but the carapace, or shield of the 
head-thorax, may be seen, after close examination, to rep- 
resent the second antennal and mandibular segments, 
