DECAPODA. 



293 



no abdominal feet ; the compound eyes are large and usually 

 sessile, and the carapace is often armed with a long dorsal 

 and frontal spine. Fig. 260 represents the Zoea, or larva of 

 the common shore crab (Cancer irroratus Say). After sev- 



Fig. 260. Zoea of the common Crab. Cancer. Much enlarged. After Smith. 



eral moults, the thoracic legs appear, the mouth -parts 

 change from swimming -legs to appendages fitted for pre- 

 paring the food to be swallowed and digested. This stage 

 in the short-tailed Decapods or crabs, is called the Mega- 

 lops stage (Fig. 261); certain immature crabs having been 

 mistaken for and described as mature Crustacea, under the 

 name Megalops. After swimming about the surface in the 

 Zoea and Megalops conditions, the body becomes more bulky, 

 more concentrated headwards, and the crab descends to the 

 bottom and hides under stones, etc. 



The development of the individual crab is, in a general 

 sense, an epitome of the development of the order. In the 

 lowest genera, as in Cuma and Mi/sis, the form is some- 

 what like an advanced Zoea, while the remarkable concentra- 

 tion of the parts headwards, seen in the crabs, is a great 



