316 CRUSTACEA. 



is the homologue of the Crustacean frontal organ. In consequence 

 of these points of agreement, "we may conjecture that the ancestral 

 forms of the Arthropodan series leading to the Myriopoda and the 

 Insecta also sprung from the Protostracan group. 



If we try to give a sketch of those ancestral forms which led on 

 from the more generally circumscribed Protostraca to the actual 

 Crustacea, and which we are accustomed to call the primitive 

 Phyllopoda, we shall have to presuppose in them a more homono- 

 mously segmented hody, and a slighter differentiation of the various 

 sections of the body, than are found in the Crustacea of to-day. Each 

 of the similar trunk-segments which together constituted the greater 

 part of the body had a pair of ventral ganglia, a pair of biramose, 

 lamellate, Phyllopod-like limbs, and perhaps also (like Peripatus) 

 a pair of nephridia. Then, as we have to regard the antennal glands 

 and the shell-glands as well as the genital ducts as transformed 

 nephridia, the very varying position of these latter ducts in the 

 different groups of Crustacea seems to indicate that we must 

 attribute to the ancestral form of the Crustacea a large number 

 of pairs of nephridia. We may perhaps assume for the limbless 

 posterior region of the body (terminal or anal segment) paired 

 furcal processes as inherited from the common ancestral form of 

 the Crustacea. The most typical Crustacean characters had, how- 

 ever, evidently already found expression in the primitive Phyllopoda 

 in the structure of the anterior region of the body. AVe find here 

 the fusion of the five anterior, limb-bearing segments (to which, 

 as a sixth, should apparently be added an anterior primary cephalic 

 segment with the eyes and the frontal organ) to form a common 

 region of the body, the dorsal integument of which, enlarged by 

 folding, formed the protective dorsal shield. Among the five pairs 

 of limbs belonging to this region, the antennules, which were in all 

 cases originally uniramose, assumed an exceptional position as bearers 

 of the more important sensory organs. The second antennae, which 

 followed them, were biramose and functioned chiefly as oars, perhaps 

 also participating in the work of mastication. For this latter 

 purpose, the mandibles lying behind the upper lip were specially 

 suited through the modification of the basal joint, while the other 

 part of the limb was retained in the Copepoda as a two-jointed 

 palp. Next came two pairs of maxillae approaching in structure 

 the subsequent trunk-limbs, and perhaps still retaining their original 

 characters in the existing Crustacea. The presence behind the 

 mandibles of a paired lower lip (paragnatha) in various groups of 



