206 
PACIFIC SCIENCE, Vol. II, July, 1948 
are closely associated with the attainment of a 
terrestrial habitat. The results of these studies 
provide evidence to show that this species is 
one which is physiologically, morphologically, 
and behavioristically transitional between a 
purely marine and purely terrestrial crab. 
It is generally acknowledged that adjustment 
to terrestrial conditions requires the ability to 
resist desiccation. In this regard it was shown 
earlier that, although 18 gills are present, the 
volume of the gills in relation to the volume 
of the body of P. crassipes is the lowest of any 
of the crabs tested (see p. 146). Pearse (1929£) 
has shown that crabs which are more highly 
adapted to a terrestrial life generally have fewer 
gills and these, by comparison, constitute a 
smaller percentage of total body volume. It was 
shown previously (see pp. 144 and 145) that this 
species withstands desiccation longer than closely 
allied but lower inhabitants on the strand. It 
was also shown earlier (see p. 184) that P. cras¬ 
sipes is found typically in places of greater 
temperature fluctuation than other crabs on the 
strand in central California. Although the desic¬ 
cation tests fail to demonstrate a significant 
difference between P. crassipes and H. nudus, the 
latter species is invariably found in places of 
lower temperature. The high littoral position 
of P. crassipes unquestionably demonstrates its 
adaptation to a range of temperature fluctua¬ 
tions wider than that of the other littoral crabs 
of the region. 
It has been pointed out that in the past the 
development of speed by animals has often been 
associated with aridity (Jehu, 1923). This 
hypothesis gains support from those crabs which 
have achieved the greatest success in conquering 
terrestrial existence. Relatively great speed is 
common to all land crabs in contrast to that of 
crabs of the lower littoral belt. To examine this 
hypothesis with respect to P. crassipes, the podo- 
meric length of the second ambulatory leg of 
this species was compared with that of H. nudus 
(Table 12). Although all the podomeres of 
P. crassipes are longer than the corresponding 
ones of H. nudus, the two distal podomeres, 
especially the dactylus, show a proportionately 
greater difference in length than do the others. 
P. crassipes , therefore, is adapted morphologi¬ 
cally for speed in a manner analogous to cur¬ 
sorial ungulates whose legs, especially the distal 
segments, exhibit a tendency to lengthen. 
The regulation of osmotic pressure within 
the body is of great importance to marine or¬ 
ganisms for entering terrestrial fresh-water 
habitats. Pearse (1931) has shown that crabs 
living continually in, or closely associated with, 
the ocean have the densest body fluids; those 
which live on land have the least saline fluids. 
Arthropods then, like vertebrates, in becoming 
adjusted to a life on land, lose some of the salt 
content of the blood. This loss suggests that 
such animals are developing mechanisms for 
maintaining the blood at a more or less constant 
salinity, regardless of changes in the surround¬ 
ing medium. The salt content of the body 
TABLE 12 
A Comparison of Podomeric Length of the Second Walking Leg between Specimens of P. 
crassipes AND H. nudus OF EQUIVALENT CARAPACE BREADTH. 
WIDTH OF 
CARAPACE 
(MM.) 
MEASUREMENTS (IN MM.) OF 
THE SECOND WALKING LEG 
SPECIES 
I, II, III 
Coxa, Basis, 
Ischium 
IV 
Merus 
V 
Carpus 
VI 
Propodus 
VII 
Dactylus 
Total 
H. nudus .... . 
27.1 
5.0 
12.5 
8.2 
6.0 
7.5 
39.2 
P. crassipes . 
27.1 
5.7 
13.9 
8.6 
7.7 
8.8 
44.7 
Ratio of 
P. crassipes 
to H. nudus . 
1:1 
1.14:1 
1.11:1 
1.04:1 
1.28:1 
1.15:1 
1.13:1 
