STRUCTUIJE AND CLASSIFICATION OF THE ARTHROFOPA. 549 



The eyes in some Crustacea are mounted on articulated 

 stalks, and from the fact that they can after injury be 

 replaced by antenna-like appendages it is inferred that they 

 represent the parapodia of the most anterior prosthomere. 

 The second prosthomei'e carries the first pair of antennae and 



Fig. 5. 



Fig. G. 



Fig. 5. — Diaf^rani of the liend of a crustacean. Triprostho- 

 nierous. F. P, frontal processes (observed in Cirrliiped nauplius- 

 larvse), probably representing the prostomial tentacles of Clisetopods; 

 e, eye ; Ant^, first pair of antennae ; Ant", second pair of antennae; 

 vid, niandidle ; mx^, m.v", first and second pairs of maxillae ; m, 

 mouth; I, II, III, the three prostlioineres ; IV, V, VI, the three 

 somites following the mouth; P, proctocerebrum ; D, deutero- 

 cerebrum ; T, tritocerebrum. (After Goodrich. ) 



Fig. 6. — Diagram of the head of a Ilexapod insect, e, eye ; ant, 

 antenna; md, mandible; ?»■*;', first maxilla; wic-, second maxilla; 

 m, mouth; I, region of the first or eye-bearing prosthonu're ; II, 

 coelom of tiie second antenna-bearing prosthomere; III. coelom of 

 the third prosthomere devoid of appendages ; IV, F, and FJ, coelom 

 of the fourth, fifth, and sixtii somites; P, protocerebrum belonging 

 to the first prosthomere; D, deuterocerebrum belonging to the 

 second prosthomere ; T, tritocerebrum belonging to the third pros- 

 thomere. (After Goodrich.) 



the third the second pair of antenna\ Sometimes this pair 

 of appendages has not a merely tactile jointed ramus, but is 

 converted into a claw or clasper. Three neuromeres — a 

 proto-, deutero-, and tritocerebrum — corresponding to those 

 three prosthomeres, are sharply marked in the embryo. The 

 fourth somite is that in which the mouth now opens, and 

 which accordingly has its appendages converted into 



