H6 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



extend outward and downward as two simple, slender tubes or ribbon-like filaments. In tlie second 

 form they divide in tlie same manner, but tlie two branches are s^nrals. (See Fig. xvii.) 



The third type, which is apparently confined to the anterior segments of the thorax, is figured 

 in his PI. Ill, Fig. 2. Here there was a series of at least 17 thin leaves arranged from above 



Flo. SVni.— A, a log of Branchinecla Undahli (from lotlipair) ; (/', ij', gills ; P, P', the 

 two parapodiiiin-like lobes, i-mnparabh- with those of the aunelid Lyareto (B), dc, 

 dorsal cirrus, acting as a gill. 



Fig. XIX. — Leg or parapodium i\i Eunice con- 

 ijlomerans, with a branched gill and the ven- 

 tral firms (ci) divided into two joints. — After 

 Ehlers. 



downwards, not spiral but fiat, somewhat as in Limulus, aiid radiating vertically from a jointed 

 epipodite or branchial arm. Thus as in forms like Calymene, Asaphus, Bronteus, etc., all the 

 limbs behind the mouth-iegion were cylindrical, ambulatory, and branchiferous, but those of the 

 pygidium were not broad and flat and adapted for swimmiug, as in Limulus. Thus the Trilobites 

 were less diflerentiated and more closely allied to the Aniu^lid tyi)e than the Eurypterida and Lim- 

 ulus. It thus seems i^robable that the three great groups of Arthropoda had each an independent 



origin from the higher worms, or fi'om extinct forms like them. 

 The insects, with the Myriopods, were independently evolved 

 •from some ChiTetopod worms of unknown affinities by way of 

 Peripatus; the Podostomata* by an independent path also 

 originated from a true branchiate Annelid, while theCrustacea 

 with little doubt, as generally conceded, arose from some 

 Annelid like form; perhaps one with dorsal cirri acting as 

 branchise, like those of Lyarete braziliensis, figured by Ehlers 

 (Mem. M. C. Z., 1887), these being remarkably like those of 

 ^"^ Branchipus and other Phyllopods.f (Fig. xviii, B.) 



FiQ. XX. — Section through the abdomen of Limu- -r, • i ± -i -i • • i ,t i ' i-, i-ii-i n 



lus; c, carapace; ht., heart; int., iutestiDo, ng., I* IS also to be boriie in miud that in the trilobites, the 



ganglion; en, axial jointed endopodite; ea: epipo- brauchiaj Were dorsal though lying uirder the dorsal wall of 



dal portion bearing the gills. ^^^ ^^^^^^ ^^^^ ilttachcd to an Cpipodal divisioU of the lilllbs, 



and so they are in Limulus, as shown by our Fig. xx. This dorsal position of the gill -plates should 

 be taken into account when comparing them with the book-leaves of Arachnids. 



* The class Podostomata includes both the Merostomata, of ■which Limulus is the living representative, and the 

 TriloTiites. 



tThe prohlem of the derivation of the jointed limbs of Arthropods from the soft non-jointed Annelid para- 

 podium is paralleled by that of the origin of the limbs of the higher vertebrates from the iins of fishes. The limbs 

 of Arthrojiods are divided into a few or many leverage systems, represented by segments or joints. There are all 

 grades of Artliropod limbs, from the uujointed parapodia-like limbs of Phyllopods, which are most Annelid-like, to 

 the imperfectly jointed swimming legs of the Nauplius of various Crustacea, and to the abdominal legs of Malacostraca, 

 up to the ambulatory legs of Decapods and of terrestrial tracheate Arthropods. These varieties of sh.ipe can be ex- 

 plained by adaptation to changed conditions resulting from a change from an aquatic to a terrestrial life, or from 

 simply swimming to walking on the bottom or to burrowing in sand and mud. On the other hand the appendages of 

 Syllid worms are often jointed, and in Eunice (Fig. xix) and Polynoe, the lower division of the parapodium con- 

 sists of two joints. 



