RESPIRATORY ORGANS IN DECAPODOUS CRUSTACEA. 467 
The Cumacea are probably degenerated from forms not far 
removed from the Archischizopod, and have only one gill 
remaining. 
The Arthostraca, comprising the two groups of Amphipods 
and Isopods, are also probably to be derived from the Archi- 
schizopod, but having from the beginning taken a different 
line of descent from the true Schizopods. The Amphipods 
(of which Talitrus is a well-known example) have a plate-like 
outgrowth serving for respiration at the base of each thoracic 
limb. This resembles the bract of Apus, and very probably 
represents the branchia of the Archischizopod, and is therefore 
the homologue of the Schizopod and Decapod gill. The 
Isopods have lost all traces of gills in their thoracic append- 
ages, this being probably owing in some degree to their general 
modifications to suit a terrestrial life. The branchial function, 
as an after development, has been undertaken by one branch 
(the endopodite) of the appendages that are used in swim- 
ming, namely, the abdominal appendages. This endopodite 
has therefore developed branchial filaments, which, however, 
bear no relation whatever to the branchial tufts on the abdo- 
minal appendages of the Stomapods. 
Thus, the positions of the respiratory organs in the different 
groups of the Crustacea are, to some extent, explained, 
although in some instances very imperfectly. I have not 
gone into the relations of the numerous groups of Decapods 
nor quoted their branchial formulae. Their relation to each 
other is, however, very fully given by Claus, and this paper 
merely offers a suggestion as to how the different formulae may 
be explained, whilst its chief purpose is to draw the attention 
of my fellow-students to an interesting field of morphological 
theory and observation. 
Before concluding, I must thank Professor Lankester for the 
help he has given me in showing me how to treat the subject, 
and in referring me to the memoirs which I have cited. 
