402 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
females when with eggs must be much less vigorous in their movements, because of 
the burden, but especially because of the necessity of having some care for the eggs. 
The consequently slower respiratory current would then afford better opportunity 
for barnacles in the free-swimming cypris stage to attach, as they are by chance 
sucked in and driven through the branchial chamber, or, if the cypris seeks out its host, 
the less active crabs will, again, be the easiest to find and to obtain attachment in. 
The females also bore more of the Balanus barnacles on their shells; and it was 
further found that about four-fifths of those crabs on which the Balanus was seen 
would also contain Dichelaspis. It was rare to find a female crab with Balanus 
that had not also Dlclidaspis. This fact is quite useful to one who is in search of 
the latter; promising crabs can be selected at a glance, even before capture. After 
ascertaining the above percentages, the collectors were asked to bring in only such 
crabs as carried Balanus barnacles. 
It is to be presumed that these gill parasites are thrown off with the cuticle of 
the gill in the molt of the crab, and that the frequent molts of young crabs would 
prevent the barnacles on their gills from becoming conspicuous either in size or in 
numbers. It would then be expected, as indeed it is observed, that this Dichelaspis 
is not found in young or in soft crabs. 
While one usually finds in an infested individual from two or three to eight or 
ten Dichelaspis , the number varies from one to as many as can be crowded into the 
branchial chambers — 500 to 1,000, or perhaps more. In one instance observed the 
gill chambers had been filled to overflowing, and one or two barnacles were found 
attached without to a maxilliped. 
This species of Dichelaspis is not peculiar 'to CaZlinectes as a host; both the 
edible stone crab [Menippe mercenaria Stimson) and the spider crab ( Libinia canali- 
culata Say) contain it, but with less frequency. It is probably present in other large 
Crustacea, lobsters as well as crabs, of other regions of the coast. 
No organic connection between the barnacle and its host is found; and, while 
histological study of the parasite has not been made, no signs of degeneration are 
noted, except as to the extent of the calcified area of the valves of the capitulum; as 
will be seen later, too, the mouth parts are well developed. The barnacle is, therefore, 
to be regarded as a space parasite, which has become adapted to this habitat because 
of the peculiar advantages it offers — protection, frequent transportation, and the 
supply of a continual current of water, from which small organisms can be gathered 
for food. The branchial chamber of the crab is, indeed, a particularly favorable 
place for life, and is made use of by various forms. Vorticellid colonies and ac.inetid 
protozoa, polyzoa — ectoproctous and endoproctous — nemerteans, etc., are found with 
greater or less frequency. The considerable variation in the size and shape of the 
calcified plates to be noted later (see tigs. 4 and 5) has probably no other significance 
than that it accompanies the degeneration of the plates, resulting from the fact 
that their function of protection is now performed by the carapace of the host. The 
relation of the calcified areas {t. sc. c.) to the whole valves ( T S. C.) is shown by the 
somewhat diagrammatic sketch (tig. 3) in which the outlines of the valves and the 
lines of growth are added, though in nature they are seen only under the microscope. 
A barnacle removed from its host, with no piece of gill left attached will live for 
a considerable time with rather infrequent changes of water. One thus kept for 23 
