May 30, 1895] 



NA TURE 



117 



fjund, the remains met with represent the exuvi* of living 

 animals that have cast their shell, rather than the tests of dead 

 individuals. In this particular deposit the appendages are 

 apparently in the position which they occupied during life, and 

 not such as would be assumed in the castoft' shells of recent 

 Crustacea. 



" Mr. Beecher mentions another interesting point, namely, 

 (hat nearly all the specimens are found with the back down, 

 ■which is explained by suggesting that, although they lived with 

 the ventral side downwards, the gases in the viscera produced 

 during decomposition were sufficient to overturn the animal and 

 allow it to be buried by the accumulation of the tine sediments 

 in the position in which it is now found. 



" The appendaijes of Triarlkiis appear now to be very well 

 made out. The antenns;, as seen in a number of specimens, were 

 simple multiarticulate flagella, which Walcott has shown extend 

 backwards to the lateral margin of the hypostome, so that they 

 occupy exactly the same position as do the tirst antenn-.t in recent 



' " Two small appendages, like simple palpi, with broad basal 

 joints, which may represent the maxilla, are seen in one of 

 Walcott's specimens, and there were probably four pairs of 

 similar cephalic appendages, besides the simple flagellate 

 antennae, more or less modified to serve as mouth-organs. 



" Each segment bears a pair of biramous appendages originating 

 at the sides of the axis, as in other trilobites. The anterior legs 

 are the longest, and the others gradually become shorter towards 

 ihepygidium. Each limb consists of two nearly equal branches, 

 the 'endopodite' and ' exojxjdite,' which may be correlated 

 with the typical crustacean primitive limb, and are well displayed 

 in the adult Afysis : in the biramose natatory-feet of the zoea of 

 the common shore-crab (Caniiiiis) ; and retained in the appen- 

 dages of the abdomen of the adult lobster [Hoiiiarin). Practi- 

 cally, these biramose limbs are reproduced along the entire series 

 of free segments. The appendages belonging to the pygidium 

 closely resemble the branchigerous feet of Apus, and may 

 evidently be correlated with typical phyllopod limbs. 



*' The first point insisted upon by all systematic zoologists — long 

 Ijefore the finding of appendages had thrown so much new light 

 upon our investigations — was that the great variability in the 

 number of the segments in trilobites was a feature which dis- 

 tinctly connected them with the phyllopoda. Bernard considers 

 of greater importance still the gradual diminution of the size of 

 the segments posteriorly, which remarkable feature the trilobites 

 share with Apus. I would also call attention to the fact that 

 those earlier trilobites which best exhibit this large number of 

 segments, such as Okiielhis, ParaJoxides, ike, are likewise re- 

 markalile for the simplicity and exact similarity of their segments, 

 being a serial repetition of one another, and even the coalesced 

 segments forming the head-shield share the same resemblance 

 with the free posterior thoracic and abdominal ones. Bernard 

 has given expression to the idea most aptly when he writes (op. 

 cil. p. 412) : — "'The adult is but the grown, not metamorphosed, 

 Jarva — grown by the continual development of segments from 

 before backwards, until at a certain stage this process becomes 

 fixed, and we have the adult Apits with a number of fixed rudi- 

 mentary segments. This fixation of a number of undeveloped 

 segments is visible also in many trilobites. 



" In the earlier forms (as OUnclliis) these rudimentar)- posterior 

 segments still remain free ; but, as a rule, they are coalesced to 

 form the plate-like pygidium so characteristic of the trilobites. 



'• Turning to the apjiendages, the simple multisegmented flagel- 

 late antenmi; are extremely characteristic of the Crustacea, being 

 met with in lowly copejxjds and highly-developeil decapods. 



" The biramose paired limbs are quite a primitive type, like the | 

 segments to which they are attached, exceedingly simple, yet 1 

 characteristic, and with the exception of the antennx- and the ! 

 four succeeding pairs of appendages, which are modified to serve 

 as mouth-organs (maxilla; and niaxillipeds), the whole series are 

 simple biramose natatory or walking-feet, such as persist still in 

 adult .\lysii and many other recent Crustacea. ' 



" The eyes in trilobites closely resemble those of other anthro- 

 pods, but vary somewhat in position, and also in development, j 

 in some genera the eyes being altogether absent, as in Ainpy.x, I 

 Ctratirui. &c. , whilst in others, like Aiglina, they are enor- ' 

 mously exaggerated in size. In some genera the eyes are hyaline, 

 the faceted surface being covered with a fine trans|>arent layer, 

 whilst in others the facets appear prominently on the surface. It 

 '■^ suggested by Bernard that the minute iiore observed in the 

 ■ad, near the compound eye in several genera (Tniiiic/eus, I 



NO. 1335, VOL. 52] 



.■l,!Uasp!s, Calymeiti, Ampy.x, Grijffil/iiJci, P/iiliipsia,ik.c.), may 

 be analogous to the pore in the head-shield of Apus, and be the 

 opening into the water-sac covering the eyes : and whilst in some 

 genera of trilobites this water-sac may have existed, it may have 

 degenerated in others, leaving the eye in contact with the outer 

 cuticle, which covered it like a thin transijarent membrane. In 

 none of the trilobites have larval eye-spots been observed. 



" Dr. Lang held the view (in 1S91 ) that if a fifth pair of cephalic 

 limbs were found comparable with the anterior antenna.-, tri- 

 lobites might then be regarded as primitive entomostraca, to be 

 derived from the same racial form as the ])hyllfipoda. 



" Walcott is of opinion that the trilobiia formed a distinct 

 branch, which diverged at a very early date from the phyllopoda, 

 and having expended its vital energy in Palaeozoic times it dis- 

 appeared. He adds : ' Probably two thousand species and one 

 hundred or more genera are known from Palceozoic strata. With 

 this great diflerentation the initial vital energy of the group 

 became impaired, and the trilobita died out at the close of 

 Palaeozoic time. 



" I willingly adopt the view that the trilobita are ancestrally 

 connected with Liniuliis ; that Limiibis may be related through 

 Hemiaspis with Etirypterus : but all the intermediate forms have 

 not yet been met with. That some ancestral Eurypterid must 

 have given rise to Scorpio cannot, I think, be doubted ; but it 

 must have been in pre-Silurian times, for Peach and Lindstriim's 

 Paheophoiius had already appeared in the Upper Silurian of 

 Lanarkshire and Gotland as a terrestrial pulmonated form, 

 while a similar land-scorpion had been discovered by Whitfield 

 in the Silurian of America. 



" The Phyllopoda deserve consideration from a geological 

 standpoint, a representative of .^/hj- (Ptotocayis Marshii)\ia\m<i^ 

 been met with in the Lower Cambrian of Vermont, U.S. 

 Some of the living genera are naked { Branchipus and Arteiiiia), 

 but in most the front portion of the body is protected by a shield- 

 like carapace (Apus), or it may be enclosed, as in Esthoia, in a 

 bivalve shell. The fossil remains of bivalved phyllopods, 

 Estheria and Leaia were described by Prof. T. Rupert Jones as 

 far back as 1862 in the Pala;ontographical Society, where he 

 defines nineteen species ranging from the Old Red and 

 Carboniferous upwards. 



'■The most ancient of these shield-bearing crustaceans, 

 originally placed with the phyllopoda and having a single 

 modern analogue (Xebalia), have now, by general consent, l)een 

 removed and placed under the order Phyllocarida, a name 

 suggested by Dr. A. S. Packard in 1S79. The fossil forms 

 referred to this order were originally studied and noticed In- 

 M'Coy, Salter, Barrande, Clar*e, and have subsequently been 

 fully described by Prof. T. Rupert Jones and myself. 



" Metschnikoff, who studied the embryology of Kehalia, con- 

 sidered it to be a ' phylloix>diform decapod.' Besides the 

 resemblance to the decapods, there is also a combination of 

 copepod and phyllopod characteristics. The type is an instance 

 of a generalised form, and is of high antiquity, having made its 

 appearance in Cambrian times, when there lived (if we regard 

 the relative size of most Crustacea, and especially that of the 

 living Nibalia) gigantic fonns. Such was the Silurian dra- 

 tiocaris ludiiisis, which was probably more than two feet 

 in length. 



" The modern Nehalia is extremely small, about I inch in 

 length, but a newly-descrilwd s|iecies, Nehaliopsis lypica, Sars, 

 measures as much as if inch, w ith the body compressed, and the 

 carapace bivalved, as in Lininadia, one of the genuine phyllopxls. 

 There is a large movable rostrum overhanging the head ; stalked 

 eyes; the cephalic portion carries two pairs of antennx and 

 three pairs of special mouth-organs (mamlibles and maxilla) ; 

 the thoracic segments bear eight pairs of short, leaf-like re- 

 spiratory-feet, which are followed by six pairs of (aMominal) 

 simple swimming-feet, four being large and two rudimentar)-, 

 while the last two segments (seventh and eighth) are destitute of 

 appendages, the body terminating in an elongated phyllo|K'Hl- 

 like caudal fork. Comjjared with Kehalia, the fossil forms give 

 evidence of an articulated rostrum ; traces of antennx : the 

 presence of a pair of strong mandibles ; of a large exjxtnded 

 shield in some, and of a folded or bivalved cara|xace in others ; 

 of the presence of seven or eight body-segments, sometime- 

 carrying branchigerous appendages, the terminal segment carr)- 

 ing a central caudal spine and two lateral shorter ones. It seems 

 highly probable that the old giant pod-shrimps (Ceralioiaris. 

 /'/Mj'rft'ar/y, &c.), whose remains occur in the Palceozoic rocks 

 from the Cambrian to the Carboniferous, are represented by the 



