INTRODUCTION vii 



as a swimming organ. The segments may or may not 

 be provided with false feet on their under surface. 



The internal antennas, antennules, are thread-like 

 limbs or " feelers," either folded under the front into pits, 

 antennulary fossae, or free and prominent ; they are 

 composed of basal joints, and terminate in one or more 

 terminal threads, flagella. 



The external antennae, antennae, are usually situated 

 near the internal antennae and outside them, nearer to the 

 eyes. They do not rest in pits, are composed of basal 

 joints, and terminate in a long thread, flagellum, usually. 

 They may be very small or very long and strong ; some- 

 times they are in the form of flat plates. The part of 

 the front where these two pairs of organs spring is called 

 the antennary region ; below this region, above the mouth, 

 is the epistomian region. 



The mouth, buccal cavity, is situated below the 

 epistomian region ; and is bounded on the sides by the 

 pterygostomian region, which is the front part of the 

 infero-lateral wall of the carapace. 



The external maxillipedes. In crabs these close 

 the mouth like two doors, the fourth and fifth joints 

 being flat. They are sometimes pediform, like feet, and 

 usually have an external appendage (exognath) ; the 

 internal part is sometimes called the endognath. On 

 the summit of the fourth joint in crabs is seen three very 

 small terminal joints, the fifth, sixth, and seventh joints. 



The legs are composed of five pairs, the first pair 

 used as hands usually, anterior legs, pincers, chilipedes, 

 and the four posterior pairs, or walking legs. The first 

 pair only may have pincers or all the legs may have them ; 

 sometimes some of the pairs are modified for various 

 duties, holding, walking, or for swimming ; sometimes 

 the last pair is very small and atrophied. 



