482 DECAPODA. 



with two blades, of which the distal is the largest, both armed 

 with stiff bristles. The posterior maxillae have a small respira- 

 tory plate (exopodite), an endopodite (palp) shaped like a 

 double blade, and two basal joints each continued into a double 

 blade. The two maxillipeds (iiixp I and mxp 2) have the form 

 and function of biramous swimming feet. The exopodite of 

 both is two-jointed and bears long bristles at its extremity ; the 

 endopodite of the anterior is five-jointed and long, that of the 

 second is three-jointed and comparatively short. 



In the six-jointed tail the second segment has usually two 

 dorsally directed spines, and the three succeeding segments each 

 of them two posteriorly directed. The telson or swimming plate 

 is not at first separated from the sixth segment ; on each side it 

 is prolonged into two well-marked prongs ; and to each prong 

 three bristles are usually attached (fig. 224). The heart (fig. 

 224 ///) lies under the dorsal spine and is prolonged into an 

 anterior, posterior, and dorsal aorta. It has only two pairs of 

 venous ostia. 



During the Zoaea stage the larva rapidly grows in size, and 

 undergoes considerable changes in its appendages which reach 

 the full Decapod number (fig. 224). On both pairs of antennas 

 a flagellum becomes developed and grows considerably in length. 

 Before the close of the Zoaea condition a small and unjointed 

 palp appears on the mandible. Behind the second maxilliped 

 the third maxilliped (inxp*} early appears as a small biramous 

 appendage, and the five ambulatory feet become distinctly 

 formed as uniramous appendages the exopodites not being 

 present. The third pair of maxillipeds and three following 

 ambulatory appendages develop gill pouches. The abdominal 

 feet are formed on the second to the sixth segments of the tail 

 as simple pouches. 



The oldest Zoaea is transmuted at its moult into a form 

 known as Megalopa, which is really almost identical with an 

 anomurous Decapod. No Schizopod stage is intercalated, which 

 shews that the development is in many respects greatly abbrevi- 

 ated. The essential characters of the Megalopa are to be found 

 in (i) the reduction of the two anterior maxillipeds, which 

 cease to function as swimming feet, and together with the 

 appendages in front of them assume the adult form ; (2) the full 



