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 Al A 
a (a 
INSECT........ TA le ia a eal es hl a eT a i | 
23 4 5 6 7 8 9 1011121381415 1617 18 19 20 21 
Crustacean... | | if 1 {.) | bid) del dda lA aie 
SS 
C T A 
eS ) 
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; CT, for the cephalothorax. 
The number of normal segments in a Crustacean has been so 
clearly and conclusively demonstrated by Milne-Kdwards that it 
is unnecessary to add here to what has already been said on the 
subject. The serics 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 | to the eyes, 
2, 3, to the two pairs of antenne ; then, in the Decapod, 4, 5, 6, 
7, 8, 9, to organs of the mouth (or mandibles, maxille, and 
maxillipeds) ; 10, 11, 12, 18, 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 Squalla 
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 pl. 24. 
* In the Tetradecapod, 4, 5, 6,7 pertain to organs of the mouth, and 
8, 9,10, 11, 12, 13,14 to feet. 
