53 
1916-17.] Experiments and Observations on Crustacea. 
It will be convenient at this point to make a few general remarks 
regarding the effect of distilled water on the animals. Reference has 
already been made to the oedema produced in two of the animals * 
immersed in one-quarter sea-water. A similar change is very common, 
but not invariable, after immersion in distilled water. Frequently the 
animal affected with oedema acquires a very un-Ligian contour : in- 
creasing considerably in length without any corresponding increase in 
breadth, it comes superficially to resemble one of the commoner Idotese. 
The elongation is due to separation of the calcified somites, the soft 
integument between these harder structures (normally hidden) being 
prominently visible on the dorsal aspect of the thorax. A flaccid Ligia 
that has died a natural death may be pulled into a similar form. The 
animal that takes on this elongated shape after immersion in distilled 
water is anything but flaccid, however. Its weight has considerably 
increased. With forced ventriflexion the integument between thorax and 
abdomen readily ruptures and some of the viscera escape along with a 
minimal amount of fluid. The fact that there is no gush of free fluid can 
indicate only that the condition is one of true oedema, in which the tissues 
as a whole have become infiltrated with water. When removed from the 
immersion-flask and placed on the table, animals in this condition are 
found to be still capable of feeble locomotion. 
Other parts may participate in the swelling. Thus, one invariably 
finds that the lower lip protrudes, while the gills, instead of lying flat 
along the ventral aspect of the abdomen, project ventrally almost at right 
angles to the body axis. Some of the gill-lamellm may be swollen into 
rounded bladders. There is much variation as regards the number and 
situation of the gills thus affected. As a rule the endopodites alone are 
swollen, but the swelling may affect exopodites ; rarely the exopodites are 
affected to the exclusion of the endopodites. The gills may be swollen in 
cases where the body is not elongated, and vice versa. 
It is plain that swelling of the gills is merely one part of a more 
general process. Nor does it seem necessary, as Miss Stewart has done, to 
refer the death of immersed Ligise to a primary change in the gills. From 
the fact that she observed swelling only in the internal gill-lamellse this 
writer is inclined to argue that these lamellse are more directly concerned 
with the respiratory function. Apart from the validity of the premises, 
one would demand more convincing proof than this. 
While death may occur in distilled water without any sign of oedema, 
i.e. while water may fail to enter the animal in sensible quantity, in every 
case salts diffuse from the animal into the water. Repeated washing of a 
