L.EMODIPODS- 



-CRUSTACEA. 



- IsoroDS. 



289 



Tlicy are found in great numbers under stones, or 



under the decaying sea-weeds thrown up on the beach 



by the tide, and when dis- 



Fig. 187. turbed leap with great 



agility. At times they occur 



in such countless swarms 



as to give the appearance of 



a thick mist at the water's 



edge as they bound into the 



air to a considerable height. 



The species we give as a 



representative of the genus 



and of the order, is the 



-See fig. 187. 



Common Sandhopper. 



2\ditrus locusta or saltatur. 



Order II.— THE L.cEMODIP0DS {Lccmodipoda). 



In this order the branchiae or gills are vesicular, as 

 in tiie Arapliipods, but are only attached to the second 

 and third, more rarely the first, thoi'acic rings. Tiie 

 segments of the thorax are six in number, and the feet 

 vary from five to seven pairs. The first is attached to 

 the head, and, as well as the second pair, are ter- 

 minated by prehensile claws, while the others are 

 provided with a huok, flexible and more or less pre- 

 hensile. The abdo-Tien is reduced to such a rudi- 

 mentary state that it is scarcely visible, being little else 

 (ban a tubercle. The females carry their ova as the 

 Amphipods do. The species belonging to this order 

 are few in number, and none of them are large. 

 Some of them have the body cylindrical in shape, the 

 legs long and very slender, and they live free and un- 

 attached, as the Skeleton Shrimps {Caprella), which are 

 found amongst marine plants, creeping along in the 

 same way as the "looping caterpillars," often bending 

 themselves back with great rapidity, and applying their 

 antenna; to various parts of their body, while in swim- 

 ming they bend the two ends of the body downwards. 



Others have the body 

 r'ie- 188. much developed, flat, 



and broad, the feet 

 thick and hooked at 

 their extremity, and 

 they live parasitical 

 upon other animals. 

 Such are the Whale- 

 lice {Cyamus), which 

 are generally found 

 attached to the bodies 

 of wliales, and are occasionally seen in such numbers 

 upon them, that the individuals so infested are recog- 

 nized at a considerable distance by the white colour 

 Ihus imparted to them by the parasites. 



As an illustration of this order we subjoin a figure of 

 Caprella linearis, the Skeleton Screw. — See fig. 188. 



OitDEK III.— THE ISOPODS {hopoda). 



The Isopods have their branchix or gills in the form 

 of large, oval, more or less membranous plates, placed 

 Vol. II. 



CiipreUa Unearis — Male and Female. 



at the extremities of six pairs of members attached to 

 the abdominal segments, and called false feet. The 

 number of true feet are seven pairs, which are almost 

 always terminated by a somewhat sharp and often 

 more or less prehensile claw. The females carry their 

 eggs under the thorax, in a pouch attached to the base 

 of the feet. They are retained there till the young 

 are hatched, and the mother then opens the pouch 

 and allows them to make their escape. This order 

 contains many species, which are distributed over a 

 wide range. They may be divided into three large 

 groups: — 



The first group are termed the Walking Isopods. 

 They have their feet fitted chiefly for walking, and 

 not adapted for swimming. Representatives of this 

 group are the Sea Centipedes {Idntea) ; the Screws, 

 as they are generally called (Asclhis), the various 

 species of which live in fresh water; and the Wood 

 lice (Onisous) which are formed exclusively for living 

 on dry land. 



The Limnori.*; belong also to this group. There 

 is only one species (jLimnoria terebrans — see fig. 189), 

 but it is well known for its destructive habit of 



Fig. 189. 



Fig. 100. 



Boring l/imnoria (Limnoria 

 terebrans). 



Sea Pill-ball {Spliajroma 

 serratum). 



boring into submersed timber. It is very small, sel- 

 dom exceeding two-tenths of an inch in length ; but 

 it is gregarious, and very abundant in some situations. 

 It is marine, and is generally found living in holes in 

 wood, which it forms for itself. Tlie ravages this little 

 creature commits in the timber forming piers, dock- 

 gates, &c., are very great. It was first particularly 

 noticed by the late Mr. Stephenson, the eminent 

 engineer, while engaged in the erection of the Bell 

 Rock Lighthouse. Since then it has been observed 

 in several other parts of our coasts, and has been the 

 cause of most serious injury to piles of wood support- 

 ing various useful erections on the shores of the sea. 

 The galleries which it forms in the wood are tortuous, 

 and run in all directions, though generally the animal 

 bores upwards at an angle of about 45°. By being 

 thus bored throughout its substance, the wood becomes 

 so disintegrated that the sea washes away its surface, 

 layer after layer, and the whole piece of timber is soon 

 destroyed. 



The second group are termed the Swimming Iso- 

 pods. The species belonging to it liavc iheir feet more 

 or less adapted for swimming, and the last segment of 

 the abdomen forms with the horizontal plates with 

 which the sixth pair of false feet are terminated a large 



caudal fin. 



2 



