HA RD WICKE 'S S CIENCE - G OS SIP. 



13 



they were in this respect really very highly organized, 

 ^nd this highly-developed specialization of the sense 

 of sight certainly proves that they ought to be placed 

 much higher among the Crustacea than we find them 

 in Haeckel's "Systematic Survey." In many species 

 of Trilobites the empty eye-sockets can be seen with 

 the naked eye, notably so in Phacops caiidatus, in 

 which each eye contained four hundred facets. Ac- 

 cording to Owen, AsapJiHS tyrannus possessed no 

 fewer than six thousand eyes ! The number of eyes 

 among the Trilobites varies considerably ; some spe- 

 cies have none at all. 



Fig. 16. Fossil King crab, from 

 coal measures of Coalbrook- 

 dale {^Bclinurus trilobitioidfs). 



Fig. 17. Tritutcleus fiin- 

 briatus. Upper Llandeilo 

 beds, Builth. 



We have already referred to the fact that the Tri- 

 lobites are peculiar to the primary rocks. Although 

 they seem to range as high as the Permian, they are 

 chiefly confined to the strata below and including the 

 Carboniferous limestone. No fewer than four hundred 

 species, grouped in fifty genera, have been described 



Fig. 18. Compound eye of fossil Trilobite {_Asa/>hiis caudaiits) 



slightly magnified. 



Fig. ig. Ocelli of ditto (magnified). 



from these formations, and new forms are still oc- 

 casionally met with. The greater number of the 

 species are of Silurian age ; those of the Devonian 

 rocks are of a well-defined character ; and those 

 from the Carboniferous limestone even more distinct 

 still. It would seem as if they reached their maximum 

 of size, as well as of variation, during the Silurian 

 period. The largest is the Asaphtts gigas, eighteen 

 inches in length, found at Llandillo. On the other 

 hand, they appear to have decreased in size as well as 

 in numbers when we reach the carboniferous rocks. 

 The genus Phillipsia, there represented, rarely includes 

 specimens more than three-quarters of an inch in 

 length. It ought to be stated, however, that we 



know little about the embryology of the Trilobites . 

 There cannot be a doubt that many of the so-called 

 species, and even genera, are larval stages in the de- 

 velopment of the same species. We have referred 

 to the common Lobster as an illustration of the 

 clearly-marked characters appertaining to the various 

 stages in the life-history of the same individual. It 

 must be remembered also that each of these stages is 

 accompanied by as many "moults"; and if we reason 

 from our general experience of the embryology of the 

 Crustacea, we must allow that the Trilobites were 



Fig. 20. Parasite of Shrimp [Bopyrus crangorimt) ; a, upper 

 side ; b, profile ; c, under side ; d, highly magnified and 

 aborted foot ; e, upper side of male Bopyrus, much smaller 

 than female ; _/, lower side of ditto ; g, part of carapace of 

 shrimp, swelling out to show presence of parasite underneath. 



affected in the same manner. The number of larval 

 stages they passed through depends upon the position 

 they attained as regard? organization. We think this 

 was much higher than Haeckel imagines, and there- 

 fore that the stages may have been numerous. It is 

 to be expected that individuals would die and be 

 buried in the muddy ooze in each of these intermediate 

 states. Thus found, what more natural than to regard 

 them as different species, and even different genera ? 

 Only a fuller knowledge of crustacean embryology 

 will clear away a good deal of the ignorant nomen- 

 clature which has gathered about these interesting 

 creatures, and it is hardly to be expected that we shall 

 ever know their accurate life-history. Barrande, who 

 had such splendid opportunities for studying the Tri- 

 lobites, and who made equally good use of them, 

 satisfied himself, in the case of no fewer than twenty 

 different species of Trilobites, that they passed through 

 larval stages, each unlike the other. In some in- 

 stances he traced them from when they must only just 



