292 
XlPHOSUKA.- 
-GRUSTAOKA.- 
-XlPHOSUKA. 
can no longer move from the position they have chosen. 
Others retain to a considerable extent their powers of 
locomotion, and some are able to change their position 
from one animal to another at will. The species be- 
Fig. 196. 
Fig. 197. 
Ari?nlus foliaceus. 
Lernsea branchialis. 
longing to this section are divided into two orders, 
SiPHONOSTOMATA and Leknjeida^. 
The first order, the Siphonostomes are represented 
by Argulus foliaceus (fig. 196) ; the second, the 
Lernmans. by Lcrncea branchialis (fig. 197). 
In the arrangement of the Crustacea, by 
M. Milne Edwards, who has done much for 
this class of animals, there, follows immediately 
after the Lernceidce the curious-looking creatures 
called the Sea Spiders. 
SEA SPIDERS {Pgcnogonidce), 
The Sea Spiders were originally classed by 
authors amongst the spiders, and it is only 
lately that they have been finally pronounced 
to be Crustaceans. Though they have no 
branchiae or gills, they do not possess the 
trachecR or pulmonary sacs for aerial respiration, 
which the spiders are furnished with. They 
appear, indeed, to breathe through the skin, the aera- 
tion of the blood being effected by the mere contact 
of the water, in which they live, with the external 
surface of the body. They have a crustaceous slender 
body of five joints, not much thicker than the lin)bs, 
which are four pairs in number, and generally very 
long. The females have a supplementary pair, mucli 
smaller than the others, bent under them and destined 
to support the eggs. 
The Sea Spiders, as their name imports, are all 
marine, and they are of small size. They conceal them- 
selves amongst the sea-weeds and corallines between 
tide-marks, and under stones within the lowest tide- 
line, and occasionally they are dredged from deep water. 
Their motions are remarkably slow, and they probably 
live on dead animal matter or small marine inseets. 
The females are apparently fewer in number than the 
males, or at least are much more seldom met with, and 
may be readily distinguished by the supplemental pair 
of legs mentioned above as destined to hold and carry 
the eggs. These are collected into globular masses 
enveloped with a thin skin or membrane, each mass 
firmly adherent to the oviferous leg, and consisting of a 
congeries of numerous round ova. The young are 
softer and longer-bodied proportionally than the adult, 
and appear to undergo considerable changes in their 
progress to maturity. Our illustration of this order of 
Crustaceans is the Nymphon gracile (fig. 198), which 
Fig. 198. 
is distinguished by its very long slender legs, these 
members being about four times the length of the bodj^ 
Sub-class III. — XIPHOSURA (Sword-tails). 
The Sword-fails, or King-crabs, are distinguished from 
all the other Crustacea by the peculiar formation of the 
mouth Instead of having mandibles and jaws, as the 
others have, the masticatory organs are the six pairs 
of thoracic feet, which are so situated as to surroiind 
the mouth. The basal portion of these members is 
armed with strong spines, which serve the purpose of 
teeth, and thus replace the ordinary organs for masti- 
cating the food. The branchim are in the form of 
numerous plates, or membranous folds placed trans- 
versely, lying one upon the other like the leaves of a 
book, and attached to the basal portion of (he abdominal 
legs. The body is composed of three parts ; the head 
and thorax united in one {cephalo-thorax), the abdomen, 
and tail. The cephalo-thorax is covered by a large 
horny buckler of a semicircular form, and on it we see 
four eyes; tw’o very small and simple, atid two larger, 
compound, and of an oval shape. The abdomen is 
covered also with a considerable-sized buckler, of a 
somewhat triangular shape, toothed at the side.s, and 
sending off from its lower edge a long, shaj'p spine, 
nearly as long in general as the whole body, and con- 
