CRUSTACEA. 



417 



function, and begin to be converted into definite masticatory appendages 

 by the reduced jointing of their palp, and the increase of their cutting 



/(Marti 



FIG. 236. SIXTH STAGE IN THE DEVELOPMENT OF GYPEIS. (From Claus.) 



MX', first maxilla; MX", f. second maxilla; /".and/"', first and second pair of 

 feet; Fu. caiidalfork; L. liver; S.D. shell gland. 



blades. By the seventh stage all the appendages have practically attained 

 their permanent form ; the second pair of maxilla? has acquired small 

 branchial plates, and the two following feet have become jointed. In the 

 eighth and ninth stages the generative organs attain their mature form. 



The larva of Cythere at the time of birth has rudiments of all the 

 limbs, but the mandibular palp still functions as a limb, and the three feet 

 (2nd pair of maxilla? and two following appendages) are very rudimentary. 



The larva? of Cypridina are hatched in a condition which to all intents 

 and purposes resembles the adult. 



Phytogeny of the Crustacea. 



The classical work of Fritz Miiller (No. 452) on the phylogeriy of the 

 Crustacea has given a great impetus to the study of their larval forms, and 

 the interpretations of these forms which he has offered have been the subject 

 of a very large amount of criticism and discussion. A great step forward 

 in this discussion has been recently made by Claus in his memoir (No. 448). 



The most fundamental question concerns the meaning of the Nauplius. 

 Is the Nauplius the ancestral form of the Crustacea, as is believed by Fritz 

 Miiller and Claus, or are its peculiarities and constant occurrence due to 

 some other cause 1 The most plausible explanation on the second hypothesis 

 would seem to be the following. The segments with their appendages of 

 Arthropoda and Annelida are nonnally formed from before backwards, 

 therefore every member of these two groups with more than three segments 

 must necessarily pass through a stage with only three segments, and the fact 

 that in a particular group this stage is often reached on the larva being 

 hatched is in itself no proof that the ancestor of the group had only three 

 segments with their appendages. This explanation appears to me, so far 

 as it goes, quite valid; but though it relieves us from the necessity of 

 supposing that the primitive Crustacea had only three pairs of appendages, 



TJ v 97 



i>. Hi. I 



