328 Shield-bearing Crustacea. 



animal ; for all the researches hitherto made (and they are 

 many) fail to detect the slightest trace of limbs in the 

 Trilobites. It is impossible, seeing the state of preservation 

 in which they occur, to suppose that in every case — in fine 

 shale, in limestone, in arenaceous mud— all traces of these 

 organs should have been lost, had they ever existed." Mr. 

 Salter considers (op. cit., p. 9) ", that they probably lay half- 

 buried in the silt, as is the frequent habit of the large Limulus, 

 or king-crab." 



The same author notices " The curious facial suture" 

 (which separates the elements composing the head, dividing 

 the cephalon into the central part, or glabella, and two lateral 

 portions (cheeks), which bear the eyes) — " a line of division 

 only faintly indicated in the Limulus, and which has, perhaps, 

 no other representative in the whole Crustacean class." 



" Limulus," Mr. Salter adds, " also shows a trace of 

 trilobation ; but it is accidental, rather than characteristic, in 

 other groups." 



There is only one key left to us by which to unlock these 

 mysterious interblendings of forms belonging to distinct orders, 

 and even classes ; it is Embryology. 



In the young state the larvae undergo repeated moults, 

 presenting, in the course of their progress to the adult, a series 

 of forms totally unlike the parent, but bearing a resemblance 

 either to the larvse, or to the adult condition of some other 

 genus. This is illustrated by the figures of Apus in our plate, 

 the larval stages (Fig. 10, a, b, c,) fitly representing some of 

 the earliest of our fossil disc-bearing Crustacea, Peltocaris, etc. 



I hope to say more upon the subject of the larval stages of 

 Crustacea in a future paper. Whether we view these remark- 

 able modifications as so many separate indications of creative 

 design, or, as resulting from the influences of food, climate, 

 locality, and a host of other circumstances, acting upon the race 

 for immeasurably long periods of time, the result we see around 

 us must be the same, namely, an endless diversity of forms in 

 every class of the animal kingdom, adapted to suit all the varied 

 exigencies of animated existence. 



EXPLANATION OF PLATE. 



Fig. 1 . The King-crab, Limulus Mohiccanus, Fabr. (dorsal 

 surface), -fth the natural size; abundant on the coasts of 

 Molucca, China, and Japan. 



Fig. 2. Limulus polgphemiis (ventral aspect), about -g-th 

 the natural size ; common on the east coast of North America.* 



* For a description of the organs of Limulus, see article " On the Seraphim 

 and its Allies," Intellectual Ojjsekvee, toI. iv., p. 233. 



