1097 



TRILOBITES. 



TRILOBITES. 



1093 



approximated to the Chitons, or at least not differing greatly from 

 them. 



It is now agreed on all handa that Trilobites are Crustaceans. 

 The names imposed on some of the genera indicate the difficulties 

 originally felt in understanding their structure. 



Thus we have Aiaphas (aaa<t>^s, 'obscure ') ; Calymme (/cexciAvrtufVi;, 

 ' concealed ') ; Paradoxui (iropaiojos, ' wonderful ') ; Cryptonymus 

 (/cpvirrw, * to conceal ' ; tvofjm, ' a name ') j and Aynostus (tityvatTTos, 

 'unknown'). 



Dr. Buckland (' Bridgewater Treatise') expresses an opinion that the 

 nearest approach among living animals to the external form of Trilo- 

 liitcs is that afforded by the genus Serolit. [IsopooA.] He thinks 

 that the most striking difference between this animal and the 

 Trilobites consists in there being a fully developed series of crus- 

 taceous legs and antenna; in the Serolii, whilst no trace of either of 

 these organs has yet been discovered in connection with any Trilobite. 

 Brongniart, he observes, explains the absence of these organs, by con- 

 ceiving that the Trilobites hold precisely that place in the Gymno- 

 branchiate Crustaceans, in which the antenna; become very small, or 

 altogether fail ; and that the legs, being transformed to soft and 

 perishable feet bearing branchiae, were incapable of preservation. 



A second approximation to the character of Trilobites occurs, 

 according to Dr. Buckland, in the Limulut, or King-Crab (Xiphoiurua), 

 and he Bees a third example of this disposition in Branchipm stagnates. 

 [BnANcniopoDA ; OHIUOCEJ-HALUS.] 



Mr. W. S. MacLeay, in his highly-interesting paper entitled 

 ' Observations on Trilobites, founded on a comparison of their struc- 

 ture with that of Crustacea,' published in Sir Roderick Murchison's 

 work on 'The Silurian System' (1839), points out the relations of these 

 creatures with the Crustacea. 



M. Milne-Edwards, in his 'Histoire Naturelle des CrustaceV (1840), 

 expresses his belief that the Trilobites ought to be arranged between 

 the Itopoda and the Branchiopoda. In all probability they belonged, 

 he thinks, to the great division Brancliiopoda. 



Burmeister, one of the latest writers on the subject of Trilobites, 

 concludes that their structural relations are with the Pliyllopoda, 

 amongst the Entomostracous Crustacea. He thus concludes his work 

 on the organisation of Trilobites [ENTOMOSTRACA] : 



" The Phyllopoda live in stagnant fresh-water, especially in ditches, 

 pools, or puddles, which are very rapidly produced after rain in the 

 early part of the year, and last only till the middle of the summer, 

 when they become dried up. During this period the animals of the 

 tribe we are describing are usually seen in numerous companies, 

 swimming about in the water at various depths, the species of Branchi- 

 pta being most frequently close beneath the surface of the water. 

 In swimming they turn their back downwards, their abdomen being 

 upwards, so that the feet touch the surface of the water. The posi- 

 tion of the eyes in Branchipus enables the animal to look both 

 upwards and downwards. Apus, on the other hand, which has im- 

 moveable eyes, can only look downwards while swimming on ita back, 

 and it must turn itself if it wishes to look upwards. But this is quite 

 natural, since in each case the animal, when in its usual position, and 

 close beneath the surface of the water, can only have ita enemies 

 below, and therefore only needs to be secured against surprise in that 

 direction. These creatures are however not much exposed to attack. 

 Their prey, which consists of other little animals living in the water, 

 they obtain during their constaut swimming motion, and it is brought 

 to the mouth by the motion of the water. Owing to this, the region 

 and cavity of the mouth in many of these animals, when preserved in 

 spirits of wine, are frequently entirely covered or filled up with 

 extraneous substances. 



" The Phyllopoda are never at rest, partially because they are entirely 

 deficient in organs by which they could keep firm hold of anything, 

 partially also because their motion of swimming produces at the same 

 time the motion of the respiratory organs, which being independent 

 of the will of the animal does not cease. I have not yet had an 

 opportunity of observing Llmnadia and Ettheria in a living state, but 

 both genera undoubtedly exist in the same manner. I am not how- 

 ever aware whether they swim on their back. Considering the 

 atliiiities of the Trilobites with the Phyllopoda, I cannot doubt for a 

 moment that their habits during life and their mode of existence were 

 similar, and I therefore conclude : 



" 1. That these animals moved only by swimming, that they remained 

 close beneath the surface of the water, and that they certainly did 

 not creep about at the bottom as M. Kloden supposed. 



" 2. That they swam in an inverted position, the belly upwards, 

 the back downwarJs, and that they made use of their power of rolling 

 themselves into a ball as a defence against attacks from above. 



' :'.. That they lived on smaller water animals, and in the absence ol 

 such on the spawn of allied species. 



" 4. That they most probably did not inhabit the open sea, but the 

 vicinities of coasts, in shallow water, and that they lived gregariously 

 in vast numbers, chiefly of one species. 



" 5 That the number of species could never have been very great 

 This is indeed proved by the mode of their appearance in the fossi 

 state, inasmuch as scarcely more than six or eight species have been 

 found together anywhere in one stratum. 



" 6. Although the number of species has not been large, the number 



of individuals was very great indeed, a fact likewise observed in the 

 iving Phyllopoda, of which we as yet scarcely know a dozen species, 

 although these are grouped iuto about six different genera. 



" 7. The great differences existing in the dimensions of the present 

 Phyllopoda according to their age, justify us in expecting such differ- 

 ences also among the Trilobites, and very large individuals of the 

 atter therefore do not indicate a seperate species, unless other differ- 

 ences are presented." 



Dr. Buckland in his observations on the ' Eyes of Trilobites,' a 

 point, as he says, deserving peculiar consideration, as it affords the 

 most ancient and almost the only example yet found in the fossil 

 world of the preservation of parts so delicate as the visual organs of 

 animals that ceased to live many thousands and perhaps millions of 

 years ago, remarks, that we must regard those organs with feelings of 

 no ordinary kind, when we recollect that we have before us the 

 identical instruments of vision through which the light of heaven was 

 admitted to the sensoriuui of some of the first created inhabitants of 

 our planet. After referring to the labours of Professor Miiller and 

 Mr. Strauss, who have ably and amply illustrated the arrangements 

 by which the eyes of insects aud Crustaceans are adapted to produce 

 distinct vision through the medium of a number of minute facets or 

 lenses placed at the extremity of an equal number of conical tubes or 

 microscopes, amounting sometimes, as in the Butterfly, to 35,000 

 facets iu the two eyes, and to 14,000 in the Dragon-Fly ; Dr. Buck- 

 land remarks that in eyes constructed on this principle, the image 

 will be more distinct in proportion as the cones iu a given portion of 

 the eye are more numerous and long ; and that as compound eyes 

 see only those objects which present themselves in the axes of the 

 individual cones, the limit of their field of vision is greater or smaller 

 as the exterior of the eye is more or less hemispherical. The same 

 principles of construction as to form, the disposition of facets, and 

 optical adaptation, are obvious iu the eyes of Trilobites. 



Back of Asaphus caiidulus (Dudley, Mus. Stokes), with the eyes, a a, 

 preserved. (Rutkland.) 



well 



n, side view of the left eye of the same, magnified. (Buckland.) 

 &, magnified view of a ponion of the eye of Culymene macrojththahnus. 

 (Hominghaus.) 



According to Dr. Buckland each eye of Asaphus caudatus contains 

 at least 400 nearly spherical lenses fixed in separate compartments on 

 the surface of the cornea, aud he observes that the form of the 

 general cornea is 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 to a creature living at the bottom ; 

 but for horizontal vision in every direction the contrivance is com- 

 plete. The form of each eye is nearly that of the frustrum of a 

 cone, incomplete on that side only which is directly opposito to the 

 corresponding side of the other eye, and iu which, if facets were pre- 

 sent, their chief range would be towards each other across the head, 

 where no vision was required. The exterior of each eye, like a 

 circular bastion, ranges nearly round three-fourths of a circle, each 

 commanding so much of the horizon, that where the distinct vision of 

 one eye ceases that of the other begins ; so that in the horizontal 

 direction the combined range of both eyes was panoramic." 



M. Milne-Edwarda, addressing himself to the subject of antennte, 

 observes that, as in Apus, one perceives no trace of those organs when 

 the Trilobites are seen from above or viewed on their dorsal aspect, 

 and that if any vestiges of those appendages exist, they will probably 



