268 ZOOLOGY. 



is 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, B <3 ) 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^. 

 tennae, the smaller pair the most anterior ; a pair of mand1-\ 

 bles with a palpus, situated on each side of the mouth ; 

 two pairs of maxillae or accessory jaws, which are flat, di- 

 vided into lobes, and of unequal size ; three pairs of foot-jaws 

 (maxillipedes), which differ from the maxillae 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 ahtennal 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 

 antennas (Fig. 221, a'). In the second or larger antennas 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, 



