" | 1 c c wv Mary 1. Rathbun: 
(314 RESP a C 
p" 
AP PENDBEX: 
On ihe Class?fication of the Crusiacea Choristopoda or "T'etra- 
decapoda ; by Jauzs D. Dasa. 
"'uE term Choristopoda, applied to the ''etradecapods, alludes to the 
subdivision of the thorax into segments, each devoted to a separate pair 
of legs; this is a prominent. pecuiiarity of the species, distinguishing 
them from all the Podophthalmia, and with rare exceptions from the 
Entomostraca. 
This division of Crustacea is subdivided by Latreille and other subse- 
quent authors into three groups, the Amphipoda, Lemipoda and Isopoda. 
Króyer has suggested that the Laemipods are essentially Amphipods in 
structure, and his investigations have shown that in the only important 
distinetion between them, that based upon the abdomen, the two groups 
are united by gradual transitions. 1n the organs of the mouth, they 
are the same —also i in having thoracic branchial appendages and in the 
position of the thoracic legs; and moreover the abdominal appendages 
conform to the same type, as is seen when the abdomen in the Caprel- 
lide is partly elongated, as in the genus Cercops of Kroyer. 
Rejecting the division Laemipoda, the number of tribes is reduced to 
two. 'Phere is however a third tribe, which. hitherto has not been re- 
cognized. It is intermediate 1n its characteristics between the Amphi- 
poss and Isopods. 
The ÁuxrzErPODA are uniformly characterized by having— 
l. The three posterior pairs of thoracic legs thrown backward and. morte . 
or less obliquely outward, and constituting one series, while the 
four anterior pairs are thrown forward and outward, in. another 
series; this arrangement may be represented by the figures 4 : 8, 
(or 2--2 : 3, as the four pairs of the first series are often in two 
sets of two pairs each). 
2. ''he branchial appendages thoracic. 
3. "The abdominal members in two sets, the three anterior pairs sub- 
natatory, the three posterior styliform—an arrangement repre- 
sented by the figures 3 : 3. 
The true Iso»o»4, on the contrary, have— 
l. The four posterior pairs of thoracic legs in the backward series, 
and three anterior pairs in the forward series—3 : 4. 
2. The branchial organs abdominal. 
3. ''he abdominal members in two sets, the 5 anterior pairs branchial (the 
first sometimes. operculitform), and only the last styliform—5 : 1. 
These are two distinct types of structure of fundamental character ; 
and any species which do not pariake of these peculiarities are inter- 
SECOND SERIES, Vol. XIV, No. 41.—8Sept., 1852, 98 
