CRUSTA< I A. 



43 



functional development of the five ambulatory appendages ; 

 (3) the reduction of the forked tclson 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 cndopodites 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 

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

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

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

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

 form. In other cases again (eg. Portunus) the Zoaea spines are completely 

 thrown off at the youngest Megalopa stage. 



There is a gradual passage from the youngest Megalopa to 

 the adult form by a series of moults. 



Some of the brachyu- 

 rous Zoaca forms exhibit 

 considerable divergences 

 from the described type, 

 more espcially in the ar- 

 mature of the shield. In 

 some forms the spines are 

 altogether absent, e.g. Maja 

 (Couch, No. 474) and Eu- 

 rynome. In other forms 

 the frontal spine may be 

 much reduced or absent 

 (Inachus and Achanis). 

 The dorsal spine may also 

 be absent, and in one form 

 described by Dohrn (No. 

 478) there is a long frontal 

 spine and two pairs of 

 lateral spines, but no dorsal 

 spine. Both dorsal and 

 frontal spines may attain enormous dimensions and be swollen at their extre- 

 mities (Dohrn). A form has been described by Claus as Pterocaris in which 

 the cephalo-thoracic shield is laterally expanded into two wing-like processes. 



The Xo.ca of Porcellana presents on the whole the most remarkable 

 peculiarities and, as might be anticipated from the systematic position of the 

 adult, is in some respects intermediate between the macrurous Zoa-a and that 

 of the Brachyura. It is characterized by the oval form of the body, and by 



31-2 



FIG. 1*5. MEGALOPA STAGE OF CRAB LARVA. 



