CRUSTACEA. 463 



stages. In the first place the paired eyes are formed at each side 

 of and behind the unpaired eye, second- 

 ly the posterior pair of maxillae is 

 formed though it always remains very 

 rudimentary. The shell gland becomes 

 fully developed opening at the base of 

 the first pair of maxillae. The dorsal 

 shield gradually grows backwards till it 

 covers its full complement of segments. 



After the fifth ecdysis the Nauplius FIG. 209. TYPICAL PHYL- 



, -11 LOPOD APPENDAGE. (Copied 



appendages undergo a rapid atrophy. f rom Claus.) 



The second pair of antennae especially ex. exopodite ; en. endo- 



u j j j .1 oodite ; br. branchial appen- 



becomes reduced in size, and the man- g age ( epj po dite). The basal 



dibular palp the primitive Nauplius portion bearing the two proxi- 

 r , ,., . . mal projections is not sharply 



portion of the mandible is contracted separated from the endopo- 

 to a mere rudiment, which eventually dite - 

 completely disappears, while the blade is correspondingly en- 

 larged and also becomes toothed. The adult condition is only 

 gradually attained after a very large number of successive changes 

 of skin. 



The chief point of interest in the above development is the 

 fact of the primitive Nauplius form becoming gradually convert- 

 ed without any special metamorphosis into the adult condition 1 . 



Branchipus like Apus is hatched as a somewhat modified Nauplius, 

 which however differs from that of Apus in the hinder region of the body 

 having no indications of segments. It goes through a very similar meta- 

 morphosis, but is at no period of its metamorphosis provided with a dorsal 

 shield : the second pair of antennae does not abort, and in the male is pro- 

 vided with clasping organs, which are perhaps remnants of the embryonic 

 hooks so characteristic of this pair of antennae. 



The larva of Estheria when hatched has a Nauplius form, a large 

 upper lip, caudal fork and single eye. There are two functional pairs of 

 swimming appendages the second pair of antennae and mandibles. The 

 first pair of antennae has not been detected, and a dorsal mantle to form 

 the shell is not developed. At the first moult the anterior pair of 

 antennae arises as small stump-like structures, and a small dorsal shield 

 is also formed. Rudiments of six or seven pairs of appendages sprout 



1 Nothing appears to be known with reference to the manner in which it comes 

 about that more than one appendage is borne on each of the segments from the 

 eleventh to the twentieth. An investigation of this point would be of some interest 

 with reference to the meaning of segmentation- 



