400 DEC APOD A. 



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

 with stiff bristles. The posterior maxillae have a small respiratory 

 plate (exopodite), an endopodite (palp) shaped like a double blade, 

 and two basal joints each continued into a double blade. The 

 two maxillipeds (inxp 1 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 com- 

 paratively short. 



In the six-jointed tail the second segment has usually two dorsal'y 

 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 ht) 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 antennse a flagel- 

 lum becomes developed and grows considerably in length. Before 

 the close of the Zoaaa condition a small and unjointecl palp appears 

 on the mandible. Behind the second rnaxilliped the third maxilliped 

 (riixp*) 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 Zoasa 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 abbreviated. The essential 

 characters of the Megalopa are to be found in (1) 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 functional development of the five ambulatory 

 appendages; (3) the reduction of the forked telson to an oval 

 swimming plate, and the growth in size of the abdominal feet, 

 which become large swimming plates and are at the same time 

 provided with short endopodites which serve to lock the feet of the 

 two sides. 



With these essential characters the form of the Megalopa differs con- 

 siderably in different cases. In some instances (e.g. Carcinus moenas) the 

 Zosea spines of the youngest Megalopa are so large that the larva appears 

 almost more like a Zoaea than a Megalopa (Spence Bate, No. 470). In other 

 cases, e.g. that represented on fig. 225, the Zosea spines are still present but 

 much reduced ; and the cephalo-thoracic shield has very much the adult 



