CRUSTACEA. U 



without appendages, while the seven anterior pair of cephalic appendages are present, 

 most of them biramous in character. In the higher Decapoda the zoea frequently 

 gives rise to a Megalops (PL I., Fig. 5), with very large, stalked eyes, and the com- 

 plete number of appendages, from which, by a series of moults, the adult form is pro- 

 duced. 



In the third type which we have to consider, the young undergo all of these 

 changes within the egg, and when hatched more or less closely resemble the adult, the 

 full number of appendages being formed. 



On hatching from the egg the young crustacean usually begins to eat, and with the 

 assimilation of food an increase in size occurs. Now the larva is enclosed in a hard- 

 ened integument, and so to accommodate this growth, the skin is periodically shed. 

 This exuviation is accomplished in different ways ; the skin or shell either splits across 

 between the segments or down the back, and the animal withdraws himself through 

 the opening thus presented. At first the new skin is very soft, but it gradually 

 becomes harder, at last acquiring its proper condition. In the young these ecdyses 

 are very frequent, and are often accompanied by the great changes in the shape and 

 appearance of the larva just described, but as the animal approaches the adult condition 

 the exuviations are less frequent, and the changes less marked. 



The classification of the Crustacea has not yet reached a satisfactory condition, 

 but for our purposes the following grouping of the sub-classes may prove convenient : 

 Sub-class I., Cirripedia ; II., Entomostraca; III., Podophthalmia; IV., Edriophthalmia. 



The Phyllopod division of the Entomostraca seems to represent the central stem 

 around which the other groups are arranged, and with which they are phylogenetically 

 related, though as yet our knowledge is not sufficient to clearly show the hues of descent 

 and the degrees of relationship of the various forms. 



