406 COMPARATIVE ANATOMY CHAP. 



Development of the eye. Each eye consists of three elements : (1) a hypodermal 

 layer, ('2) an ectodermal imagination, and (3) a nerve mass. 



The hypodermal layer becomes many-layered ; some of its elements combine to 

 form groups, each of which consists of 8 cells, 4 of which (Semper's cells) secrete a 

 cuticular corneal facet, while the 4 others produce the crystalline cone. 



The ectodermal invagination (optic fold) deepens in the Nauplius stage 

 and is constricted off in stage (G) as a solid mass of cells. This mass develops 

 into a fold, opens outward and upward, with an outer and inner wall several layers 

 thick. The outer wall becomes connected with the layer of the crystalline cones, its 

 cell elements form groups of 6 to 8 and become the retinular cells. The inner wall 

 yields the nervous connections of the retinular layer with the ganglion opticum. 



The ganglion opticum arises like any other ganglion of the central nervous system 

 as an ectodermal thickening. 



Between the hypodermal layer and the optic fold, mesodermal elements penetrate 

 and grow, uniting to form a fenestrated mass which secretes a large quantity of 

 pigment. 



XV. The morphological significance of the most important Crus- 

 tacean larval forms, and the Phylogeny of the Crustacea. 



That the Crustacea form a single class is evident to the student of comparative 

 anatomy and ontogeny. It is probable that they can ultimately be traced to one 

 racial form. Basing our conjectures for the present simply on the comparative 

 anatomy and classification of the group, we feel justified in describing this racial 

 form as follows. 



The original Crustacean was an elongated animal, consisting of numerous and 

 tolerably homonomous segments. The head segment was fused with the 4 subsequent 

 trunk segments to form a cephalic region, and carried a median frontal eye, a pair 

 of simple anterior antennae, a second pair of biramose antennre and 3 pairs of biramose 

 oral limbs, which already served to some extent for taking food. From the posterior 

 cephalic region proceeded an integumental fold which, as dorsal shield, covered a 

 larger or smaller portion of the trunk. The trunk segments were each provided with 

 one pair of biramose limbs. Besides the median eye there were 2 frontal sensory 

 organs. The nervous system consisted of brain, resophageal commissures and 

 segmented ventral chord, with a double ganglion for each segment and pair of limbs. 

 The heart was a long contractile dorsal vessel with numerous pairs of ostia seg- 

 mentally arranged. In the racial form the sexes were separate, the male with a 

 pair of testes, the female with a pair of ovaries, both with paired ducts emerging 

 externally at the bases of a pair of trunk limbs. The excretory function was carried 

 on by at least 2 pairs of glands, the anterior pair (antennal glands) emerging at the 

 base of the second pair of antennas, the posterior (shell glands) at the base of the 

 second pair of maxillae. The mid-gut possibly had segmentally arranged diverticula 

 (hepatic iiivaginations). 



This conjectural racial form shows a considerable correspondence with the Annelida, 

 and this correspondence would be increased if it could be satisfactorily proved that 

 the biramose limbs of the Crustacea with its exo- and endopodite answered to the 

 dorsal and ventral parapodia, 1 and the epipodial gills of the Crustacea to the dorsal 

 gills (of the dorsal parapodia) of the Annelida. The setiparous glands of the Annelida 

 woxild correspond with the segmental leg glands of certain Crustacea (Phyllo2wda). 



1 It must here be remembered that the question whether in the Polyc'ha'M the uniserial 

 or the biserial arrangement of the parapodia is the original is not yet satisfactorily settled. 



