ARTHROPODA. 71 



gnathites : (2) "Zoea" elongate, segmented ; thorax with a dorsal 

 spine ; two sessile eyes ; abdomen as long as the body ; legs rudi- 

 mentary: and (3) " Megalopa" flattened, segmented, no dorsal 

 spine ; two pedunculate eyes ; abdomen much diminished, parti- 

 ally bent under the body ; five pairs of legs. There are inter- 

 mediate forms, and, not to be referred to the above, are Alima, 

 Erichthus, and Phyllosoma, which are also said to be larvae the 

 two former of Squillidae, the latter of Scyllaridse. The lower 

 Crustacea do not pass the Zoea-stage ; and some go through the 

 Nauplitts-stage in the egg. 



The sexes sometimes differ considerably, and there are occa- 

 sionally two forms of males. Cymothoa and some allied genera 

 are said by Bullar not to have distinct sexes. 



Owing to the extreme modifications in this class it is conve- 

 nient to divide it into six subclasses. Some of these, however, and 

 many of the minor groups, pass into one another with scarcely 

 any line of differentiation. The grades of the divisions are 

 therefore variously estimated by authors, and the names are not 

 always conterminate. 



Adult parasitic ; if attached by the feet, . 

 then the feet rudimentary. 



Sexes united CIRRIPEDIA. 



Sexes distinct EPIZOA. 



Adult free, or, if parasitic, attached by 



the feet, the feet jointed. 

 With respiratory organs. 



Carapace in one or two pieces. . . ENTOMOSTRACA. 



No carapace EDRIOPHTHALMA. 



Eyes pedunculate PODOPHTHALMA. 



No respiratory organs PODOSOMATA. 



Subclass I. CIRRIPEDIA. 

 PECTOSTRACA. 



Body furnished with a mantle, and enclosed in a many-valved 

 carapace. Abdomen rudimentary or obsolete. Feet in the form 

 of cirri. Mostly hermaphrodite. 



The adult is attached to some foreign body by the anterior 

 extremity of the head, which is of large size, and almost always 

 enveloped in a carapace containing also the rest of the body. 



