12 



HARD WICKE 'S S CIENCE - G SSI P. 



thusiasm, believed he had detected evidences of 

 legs on the under side of some specimens, but others 

 thought these to be the remains of " calcic arches. " 

 It may be, however, that the extinct Trilobites really 

 represent a defunct order, and as such we usually find 

 them arranged in systematic works on Zoology. In 

 that case they would come in as "missing links" 

 between the Isopoda, of which the common Wood- 

 louse (Onisciis) and the Shrimp-parasite (Bopy}-tcs) 

 are familiar types, and the Merostotuata, of which 

 the well-known " King-crabs " {Li in ii Ins) are 

 examples. The larval state of the higher classes in 



Fig. 13. Asaphus caiidatus. 



Fig. 14. " Dudley Locust " 

 or Trilobite (Calymene 

 Bliciiiefibachit). 



the same order frequently resembles the adult condi- 

 tions of the lower. In the Crustacea a very large 

 number of genera are alike in the youngest state. 

 From its resemblance to the adult condition of one 

 of the lowest of the crustaceans called Nauplius, this 

 state is usually called the ' ' Nanpliiis stage. " No other 

 group of animals passes through so many metamor- 

 phoses before reaching maturity, and each of these 

 is so well marked off from the rest, that it might 

 be regarded as a generic type. Indeed, in many 

 cases, genera have been founded on these distinc- 

 tions, so that the same animal, at different periods of 

 its life, was regarded not only as a distinct species, 

 but often as belonging to another genus. The young of 

 the common lobster, for instance, passes through at 

 least wastages, which are so unlike each other that only 

 careful observation has settled they are not different 

 animals. Even when it has reached the adult condi- 

 tion, a lobster is so unlike what it will be when full- 

 grown, that it might be set down as belonging to 

 another genus. It is as if we knew nothing of the 

 metamorphoses of the Butterfly, and therefore had 

 mistaken the caterpillar and chry.salis for animals 

 belonging to groups widely separated from the 

 winged insect. 



The young of the Lininbis, or King-crab, greatly 

 resembles the adult Trilobites. As the King-crabs 

 succeeded the latter in geological time, it may be that 



it was due to the Trilobites having been "advanced 

 a stage." It will be seen that a species found in the 

 ironstone nodules of Coalbrookdale, called Belinnrus, 

 more nearly resembles one genus of the Trilobites 

 [Trinndeits) than the King-crabs of our own days. 

 Again, the female Bopyrus (fig. 20), which parasiti- 

 cally attaches itself to the inner surface of the carapace 

 of the Shrimp, has a rude resemblance to the seg- 

 mented body of some of the less highly-organized 

 Trilobites. The fact of its being a parasite shows 



Fig. 15. Under surface of recent King-crab (Z./ww/«.f). 



that it must have undergone bodily transforma- 

 tion. The figures will show that the Trilobites find 

 their natural history place between the groups above 

 named. Haeckel, however, places them among the 

 "gill-footed crabs" {Brac/iiopoda), of which the 

 water-fleas are familiar examples. We do not know 

 on what grounds this is done, for no breathing or 

 locomotive organs have as yet certainly been found, 

 although thousands of specimens of all the genera 

 have been carefully examined on their under sides. 

 Again, the compound eyes of the Trilobites show that 



