202 



XiriiosuuA." 



-CEUSTACEA.- 



-XlPIIOSURA. 



can no longer move from the position they have chosen. 

 Others retain to a considerable extent tlieir ijowers of 

 locomotion, and some are able to change their position 

 from one animal to another at will. The species be- 



rig. 19(t. Fig. 197. 



Argiilus fuliaceus. 



Lernaeft brand) ialis. 



longing to this section are divided into two orders, 

 SipiiONOSTOMATA and Lern.sid.-e. 



The first order, the Sipfionoslomes are represented 

 by Argulus foliaceus (fig. 19G) ; the second, the 

 Lernauns. by Lcrnasa hranchialis (fig. l'J7). 



In the arrangement of the Crustacea, by 

 M. Milne Edwards, who has done mnoh for 

 this class of animals, there follows immediately 

 after the Lerntmla the curious-looking creatures 

 called the Sea Spiders. 



SEA SPIDERS (Pycnogonickc). 



The Sea Spiders were originally classed by 

 authors amongst the spiders, and it is only 

 lately that thej' have been finally pronounced 

 to be Crustaceans. Though they have no 

 branchiiE or gills, they do not possess the 

 trachem or pulmonary sacs for aerial respiration, 

 which tlie spiders are furnished with. They 

 appear, indeed, to breathe through the skin, the aera- 

 tion oi' 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 limbs, 

 which are fom' pairs in number, and generally very 

 long. The females have a supplementary pair, much 

 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 insects. 

 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 suiiplemental 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. Tlie 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 {d^. 198), which 



Fig. 198. 



Kyraplion gracilo. 



is distinguished by its very long .slender legs, these 

 members being about four times tlie length of the body. 



Sub-class III.— XIPIIOSURA (Sword-tails). 



TllF, Sword-fails, or King-crabs, arc 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 surround 

 the month. The basal portion of these members is 

 armed WMth strong spines, which serve the purpose of 

 teeth, and thus replace the ordinary organs for masti- 

 cating the food. The branchire are in the form of 

 numerous plates, or membranous folds placed trans- 

 versely, lying one upon the otlier like the leaves of a 



book, and attached to the basal portion of the abdonn'ual 

 legs. The body is composed of three parts ; the head 

 and thorax united in one {ccphalo-thorax), the abdomen, 

 and tail. The ccphalo-thorax is covered by a large 

 horny buckler of a semicircular form, and on it we see 

 four eyes; two very small and simple, and 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 tlie sides, and 

 sending oft' from its lower edge a long, sharp spine, 

 nearly as long in general as the whole body, and con- 



