194- ORGANS OF LOCOMOTION. 



tail, which are so evident in the Lobster, are more con- 

 centrated in the Crabs ; and in the singular animals 

 called King Crabs (Limuli), common on the shores of 

 warm countries, a complete concentration of the abdo- 

 minal pieces takes place, a broad shield, as solid as the 

 carapace of the Crab, being substituted for the ring-like 

 plates. These variations of form, from the most perfect 

 separation of parts to the most complete union, offer 

 an interesting study ; but the change must be traced 

 through an extensive series of genera and species. 



The organs of locomotion are very different in the 

 different groups. Some Crustacea are adapted for 

 swimming, others for crawling, and others, again, lead 

 a nearly sedentary life, as parasites on other animals, 

 often on fishes, and, in many cases, on the larger Crus- 

 tacea themselves. The Crabs afford us an instance of 

 the greatest compactness in the body, the segments of 

 the head being minute, and often concealed under the 

 thorax, and those of the abdomen also of small size, and 

 coiled up under the ample shield of the same portion ; 

 so that the whole body seen from above resembles a box. 

 In this tribe, five pairs of legs belonging to the thoracic 

 portion, are largely developed, the first pair being con- 

 verted into claws; and the creature can move with 

 great ease and considerable speed on land, or crawl 

 along the bottom of the sea. But its motion, owing 

 to the position of the legs, is either sideways or back- 

 wards : it cannot move in a forward direction. In 

 many Crabs, especially in those that frequent deep 

 water, the last pair of legs have their terminal joints 

 very much widened and flattened' in fact, converted 



