14 



HARD WICKE 'S S CIENCE - G OSSIE . 



have escaped from the egg to the fully-developed and 

 mature state. In the first instance they had no joints 

 to the body, and therefore resembled one of the cara- 

 paces of the "Water-fleas"; in the last they pos- 

 sessed ring-covered bodies, movable tails, and com- 

 pound eyes. This proves that, although in their 

 young states Trilobites resembled the Ostracoda, in 

 their adult life they had proceeded much further ; so 

 that Haeckel's classification is thus proved to be 

 incorrect. We ought to add that, parallel with the 

 instance of the development of the Lobster, all the 

 above changes noted by Barrande in the Trilobites, 

 occurred before the animal had attained a tenth part 

 of its full size. In Lyell's " Manual of Geology " the 

 student will find engravings of the Triimdens in three 

 stages, each of which appears specifically distinct 

 from the other. Another skilled observer of the Tri- 

 lobites was Burmeister, who was strongly of opinion 

 that all of them underwent metamorphoses. This 

 fact ought to be a warning against the careless manu- 1 

 facture of " species." In the case of fossils less care 

 has been taken in this respect than with living animals, 

 and, in many instances, some of those who have 

 christened species were geologists rather than natural- j 

 ists. The slightest differences have been sufficient ! 

 to warrant a new specific name, and thus it is more 

 than possible that the various stages in the life-history j 

 of one species may be illustrating our manuals as 

 distinct genera and species ! Even with regard to 

 sex in adult individuals, little or nothing is known ; : 

 although among nearly all the Crustacea these differ i 

 so extremely. Owen remarks that the difference in 

 the head-plate and the terminal spines of the tail in 

 the two so-called species named Asaphus caiidaius and j 

 Asaphus longicaudatiis, may only be due to difference 

 of sex ; the inference, therefore, is that these two 

 species represent the male and female of only one. j 



The earliest Trilobites (Agnostus, &^c.) are usually 

 the simplest in structure, so that these animals are 

 not an exception to the general palceontological rule 

 that the simpler always precede the more complex 

 species of the same genus or class. Agnostiis is usually 

 found in large shoals, something after the manner in 

 which the carapaces of the ancient water- fleas are 

 met with in some of the coal-measure shales. Owen 

 suggests that this disposition of Agnostiis is "as if it 

 were the larval form of some large trilobite." The 

 young of all Crustacea usually associate together in 

 shoals, and this suggestion might therefore be reason- 

 ably taken in 'consideration with what has already 

 been said on the subject. 



The compound eyes of Trilobites are usually thickly 

 placed on raised halfmoon-shaped ridges, and the fact 

 that the sockets are so well preserved, speaks plainly 

 of the quiet way in which the fine mud was deposited 

 in which tlie animals were buried and ultimately 

 fossilized. Dr. Buckland spoke of these ridges as 

 being ' * like a circular bastion, ranging nearly round 

 three-fourths of a circle, each commanding so much 



of the horizon that where the distinct vision of one 

 eye ceased, that of the other began." He also veiy 

 sagaciously referred to the form of the ridges and 

 their position on the head-shield as " peculiarly 

 adapted to the uses of an animal destined to live at 

 the bottom of the water : to look downwards was as 

 much impossible as it was unnecessary for a creature 

 living at the bottom ; but for horizontal vision in every 

 direction the contrivance is complete." We cannot 

 refrain from further quoting a well-known passage 



Fig. 21. Simplest kind of Trilobite (Agnosites pisiformis). 



from the same author, in which a logical inference is 

 drawn from the structure of the eyes of Trilobites. 

 ' ' The results arising from these facts are not confined 

 to animal physiology ; they give information also re- 

 garding the condition of the ancient sea and the 

 ancient atmosphere, and the relations of both these 

 media to light, at that remote period when the 

 earliest marine animals were furnished with instru- 

 ments of vision in which the minute optical adapta- 

 tions were the same that impart the perception of 

 light to crustaceans now living at the bottom of the 



sea With regard to the atmosphere, we infer 



that had it differed materially from its actual condi- 

 tion, it might have so far affected the rays of light that 

 a corresponding difference from the eyes of existing 

 crustaceans would have been found in the organs on 

 which the impressions of such rays were then received. 

 Regarding light itself also, we leam from the resem- 

 blance of these most ancient organizations to existing 

 eyes, that the mutual relations of light to the eye, and 

 of the eye to light, were the same at the time when 

 crustaceans endowed with the faculty of vision were 

 first placed at the bottom of the primeval seas, as at 

 the present moment." 

 I That the Trilobites were bottom-feeders and 

 ■ haunters, there can be little doubt. The late Mr. 

 ' Salter, than whom no geologist was better acquainted 

 i with Trilobites, was of opinion that they not only 

 : lived there, but fed on the organic mud, something 

 ! after the manner of earth-worms. The simple struc- 

 ( ture of their mouths, and the absence of aM/^««(? or 

 i feelers, indicate such a habit. The inexorable limits 

 I of space, however, compel us to postpone a further 

 ' consideration of this interesting subject to another 



chapter. 

 1 {To l;e continued.) 



Colour of Birds.— In addition to the white 

 specimens of birds specified by A. P., I have a hedge- 

 sparrow quite white, except parts of the pnmary 

 feathers.—^. .S". IVesl ey. 



