386 



SCHIZOPODA. 



eye. The heart has in the majority of cases only one or two (Brachyura) 

 pairs of ostia. 



A IS, 



1 C/I v vi i 



y t^ 



/ ex^. 



mxp.i 



I tt. 



FIG. 211. THE APPENDAGES OF A CKAB 



At. I. first antenna; At. II. second antenna; md. mandible (without a palp); mx. 1. 

 first maxilla; mx. 2. second maxilla ; mxp.l. first maxilliped ; mxp. 2. second maxil- 



liped. 



ex. exopodite; en. endopodite. 



The Zoaea larva, though typically developed in the Decapoda, is 

 not always present (e.g. Astacus and Homarus), and sometimes occurs 

 in a very modified form. It makes its appearance in an altered garb 

 in the ontogeny of some of the other groups. 



The two Malacostracan forms, amongst those so far studied, in 

 which the phylogenetic record is most fully preserved in the onto- 

 geny, are Euphausia amongst the Schizopods and Penseus amongst 

 the Decapods. 



Schizopoda. Euphausia leaves the egg (Metschnikoff, No. 468 9) as a 

 true Nauplius with only three pairs of appendages, the two hinder biramous, 

 and an unsegmented body. The second pair of antennas has not however 

 the colossal dimensions so common in the lower types. A mouth is present, 

 but the anus is undeveloped. 



After the first moult three pairs of prominences the rudiments of the 

 two maxillse and 1st maxillipeds arise behind the Nauplius appendages 

 (fig. 212). At the tame time an anus appears between the two limbs of 

 a rudimentary caudal fork ; and an unpaired eye and upper lip appear in 

 front. After another moult (fig. 212) a lower lip is formed (UL) as 

 a pair of prominences very similar to true appendages ; and a delicate 

 cephalo-thoracic shield also becomes developed. Still later the cutting blade 

 of the mandible is formed, and the palp (Nauplius appendage) is greatly 

 reduced. The cephalo-thoracic shield grows over the front part of the 

 embryo, and becomes characteristically toothed at its edge. There are also 

 two frontal papillae very similar to those already described in the Phyllopod 

 larvae. Rudiments of the compound eyes make their appearance, and 

 though no new appendages are added, those already present undergo further 



