2384 J.D. Dana on the Homologies of Insects and Crustaceans, 
ments in the two types, that of the Insect or highest Insectean, 
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 ab- 
domen; C-T, for the cephalothorax. 
he number of normal segments in a Crustacean has been so 
clearly and conglusively 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, 8, to the two pairs of antennz; then, in the Decapod, 4, 5, 6, 7, 
8, 9, to organs of the mouth (or mandibles, maxillz and maxilli- 
peds); 10, 11, 12, 18, 14, to feet; and 15 to 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: aud 
this is the nearest approach in Decapods to that duplication of 
the pairs of legs to each segment which occurs in the Iuli and 
some other related Myriapods,’ 
s the true normal limit of the head in an animal is deter- 
mined by the fact that this part includes the senses, mouth, and 
mouth appendages, (for this is demonstrated by the principles of 
cephalization already explained, if not established on other 
grounds,) the head in the Decapod includes nine segments, and 
the thoraz, five, although there is no constriction of the body to 
make the division obvious to the eye. : 
The relation of the Insect-type to the Decapod is at once ap- 
parent from a comparison of the two lines in the preceding dia- 
gram. Supposing the parallelism rightly presented, the following 
facts are to be noted. 
1. The Insect-type wants the 8 posterior segments of the 
Crustacean. . 
2. The head and thorax together of the Insect-type have the 
same number of segments (nine) as the head alone of the De 
capod. 
3. The head and thorax of the Insect-type contain half of 1ts 
1 In the Tetradecapod, 4, 5, 6, 7, pertain to organs of the mouth, and 8, 9, 10, 11, 
1 The writer hes that the mul f the Phyllopods 
writer multiplication o in 
might be as 0s thee bo part oe eure sof fect bacomng separate Bo = 
ment, and that the branches iar tetionded Whe double feet of the Iuli; but as the 
_members in these multiplicative types appear often (if not always) to have the full 
Bamber of has 8, this view does not appear to be tenable. 
