16 Prof. J. D. Dana on the Homologies of 



II. — On the Homologies of the Insectean and Crustacean Types. 



By James D. Dana. 

 In a note to the article on Cephalization (Annals, 1863, vol. xii.), 

 at page 193 a brief statement is made by the writer on the 

 relations between the structures of Insects and Crustaceans. 

 The following diagram and explanations will make the subject 

 more intelligible : — 



C T A 



Insect i M I I I I i I i I I I M I I I I 



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 



Crustacean.. | | | | | | | | | | | | | | | | | I | I | | 



V ^ ^ i^ .^ -V ^ 1 



C-T 



The diagram presents to the eye the succession of normal 

 segments in the two types — that of the Insect or highest Insec- 

 tean, and that of the Decapod or highest Crustacean (including 

 Crabs, Lobsters, &c.). The spaces between the vertical lines 

 stand for the segments, which are numbered from 1 to 21. 

 C stands for the cephalic portion or head ; T, for the thorax ; 

 A, for the abdomen ; C T, for the cephalothorax. 



The number of normal segments in a Crustacean has been so 

 clearly and conclusively demonstrated by Milne-Edwards that it 

 is unnecessary to add here to what has already been said on the 

 subject. The series and its subdivisions are illustrated in the 

 line above, opposite Crustacean : fourteen segments are shown 

 to belong to the cephalothorax, and seven to the abdomen. It is 

 established beyond all doubt that each segment corresponds to 

 a single pair of members, as follows : — number 1 to the eyes, 

 2, 3, to the two pairs of antennae ; then, in the Decapod, 4, 5, 6, 

 7, 8, 9, to organs of the mouth (or mandibles, maxillae, and 

 maxillipeds) ; 10, 11, 12, 13, 14, to feet; and 15-21 to the 

 abdomen*. 



The abdominal members in all Decapods which have them, 

 and four or more posterior pairs of thoracic members or feet in 

 degradational forms of Decapods (as in Gastrurans or the Squilla 

 group and in Schizopods), are two-branched, or have two jointed 

 terminations proceeding from the second segment : and this is 

 the nearest approach in Decapods to that duplication of the 



carpels of Drimys by their margins into one, we have exactly the ovary of 

 Cinnamodendron with its sessile stigmata and five lines of placentation, as 

 shown in pi. 24. 



* la the Tetradecapod, 4, 5, 6, 7 pertain to organs of the mouth, and 

 8,9, 10,11, 12, 13, 14 to feet. 



