THE CRAY-FISH. 



207 



According to Huxley, the principal difference is to be observed in the gills, of which there are 

 twenty on each side, sixteen belonging to the limbs and four fully-developed gills attached to the 

 side of the thorax. Six of the former he calls podobranchice, or foot-gills, because they are attached 

 to the protopodite, or first foot-joint, the other ten arthrobranchice, or joint-branchiae, because their 

 origin is on the joint of the leg where it unites with the thorax ; lastly, the four on the sides are 

 called by him pleurobranchice, because they spring from the part of the thoracic 

 somite or body-ring known as the plenron, or side piece, or epimeral portion of 

 the segment. 



The River Cray-fish (Potaniobius astacus) is largely caught, and when fresh 

 boiled is a dish not to be despised. It is largely imported into London for gar- 

 nishing dishes with. The writer has with a friend taken over 900 River Cray- 

 fish in the Thames and Severn Canal in Gloucestershire in a single evening be- 

 tween eight and twelve, with a series of simple scale-like nets, baited with liver. 



The Cray-fish is one of those forms which is peculiarly interesting to the 

 zoologist, as, according to the experiments of Ratke, it passes through its earlier 

 metamorphosis in the egg, a circumstance which led Prof. West wood in 1835 to 

 doubt the correctness of Vaughan Thompson's discoveries as to the series of 

 changes which the young of most species of Crustacea undergo after they quit the 



egg- 



The most remarkable thing with regard to the genus is that, notwithstanding 

 its inability to survive in salt or even brackish water conditions, its geographical 

 distribution is wider than that of any other living Crustacean. Thus we find that 

 representative species of the Astacus Jluviatilis (which it seems, according to 

 Huxley and others, we shall have in future to call Astacus torrentium) exist 



over the whole of Europe, save Sweden and Norway and Scotland, and that four T 



. Fig. 25. 



other species inhabit the rivers which drain into the Caspian and the Black Sea. 



Two others belonsr to Japan and to the basin of the Amur, which sheds its water ia, section of eye, showing 



the several layers (rt A) 



into the Pacific. The Astaci occur again in the livers of North America west c ? I ?, 1>08i " ! '' i '; 10 1 H' 4 , 1 1 , t '"^- s 



oi tiic c} tf yrt_(tLij iii.ijf in* 



of the Rocky Mountains flowing into the Pacific, and the Cambari on the trom the comcf lc to lc ti)e 

 eastern or Atlantic side. It is more wonderful still that, separated by a wide 



. . -101 



equatorial belt, Parastacidce, or representative iorms, occur 111 the Southern 

 Hemisphere, in New Zealand, Australia, Madagascar, and South America. The biggest of the 

 Cray-fishes which attains a length of more than fifteen inches, being as large as a 

 full-sized Lobster, belongs to the Murray River, South Australia. The strangest 

 are the genus Eiigceus of Tasmania and the Cambarus of the United States, and 

 Parastacus pilimanus from Brazil, which live habitually on land in burrows 

 which they excavate in the soil. This is the only family of Macroura known 

 to me which quits the water for dry land. Its geological history is as long as 

 its geographical distribution is wide, for its ancestry can carry back their 

 lineage to Pseudastacus pustulosus and Eryma modestiformis in the Jurassic 

 rocks of Solenhofen in Bavaria. 



The eyes in the higher Crustacea, like those of insects, are exceedingly 

 complex structures (Fig. 25), composed of a great number of separate lenses 

 closely compacted together, each having its cornea, its crystalline cone or lens, 

 Fig. 26. EYE OP TKI- its pigment, and its nerve-fibre connecting it with the optic nerve. They 

 the eye^Thl labeiia- P resent everv variation, however, between this compound eye in the Decapoda 

 greUraunifte<L lenses> down to the simple eye spot in the Entomostraca, whilst in some forms, as in 



Limulus, both simple and compound eyes are present on the same head-shield. 

 These compound eyes existed far back in geological time, and may be seen most beautifully pre- 

 served in the heads of many Silurian Trilobites, notably in the genera JZglinci, Phacops (Fig. 26), and 

 Dalmannia, whilst Pterygotus, like Limulus, had both compound eyes and simple ocelli. The eyes 

 are the most constant and persistent organs possessed by the Crustacea as a class ; indeed, if we except 

 certain parasitic Isopodous forms and the Cirripedia and Rhizocephala, we shall find that the faculty 



STHUCTURE 

 OF EYE OF LOBSTEK. 



of "hesurfaceoic the eye; 



an( * le< sect i n * same. 



