396 



COMPARATIVE ANATOMY 



CHAP. 



is especially the case with the 1st maxillipedes, and the rudiments of the last 2 

 ambulatory feet. The Phyllosoma shows in a specially striking manner the character 

 of a pelagic larva. 



Development of the Brachyura. 



A very characteristic Zosea larva (Fig. 267) is hatched from the egg. Its dorsal 

 ^. shield is marked by the 



possession of long spine-like 

 processes, among which a 

 frontal, 2 lateral and 1 dorsal 

 are never wanting. All the 

 head limbs are present. Of 

 the thoracic limbs we find 

 only the 2 anterior pairs of 

 maxillipedes ; the other thor- 

 acic limbs, as well as that 

 part of the thorax to which 

 they belong, are wanting or 

 are only found in their first 

 rudiments. The abdomen is 

 segmented, and ends in a 

 fork, but has no appendages. 

 In the later Zosea stages 

 the 3d pair of maxillipedes 

 appears, the 5 ambulatory 

 feet arise as imiramose limbs 

 (without exopodites), and 

 the pleopoda begin to form. 



FIG. 267.-Zoaea of Maja, after its moult (after Glaus), h, Heart. The Zocea swims principally 



by means of the 2 biramose 



anterior pairs of maxillipedes, and also by means of the abdomen which, in com- 

 parison with that of the adult, is 



elongated and well developed. 



The older Zocea changes into a 



Megalopa larva (Fig. 268). 



This larva, but for the more 



strongly developed abdomen and 



the pleopoda, already resembles the 



sexual form. The maxillipedes and 



ambulatory feet appear as in the 



adult condition, and it must be n 



specially remarked that the ambu- /A 



latory feet are never biramose, and 



that the Brachyura thus do not 



pass through any Schizopoda 



stage. 



Through many moults the Mega- 

 lopa is gradually transformed into 



the sexual form. 



We need say only a few words 



about the development of the Fia 268. Megalopa larva of a Portunus with abdo- 



other Malacostraca. The Lcptos- men straightened out, dorsal aspect (after Glaus). 



