March 2, 1896.1 



KNOWLEDGE. 



69 



discovery was made ; and it showed that each of the eight 

 thoracic segments bore a pair of cylindrical-jointed legs, 

 adapted for wallcing rather than swimming. About the 



Fio. 



-Under side of Asap/ius platiicephalus, showing the 

 thoracic legs. (After Billings.) 



same time Dr. Woodward discovered a small palp, not un- 

 lilfe the palps of the jaw of an insect, lying close up against 

 the hypostome of another specimen of the same species. 



But in spite of such discoveries there were still sceptics 

 who denied that these were really limbs, and looked upon 

 the " legs " as mere thickenings of the under-surface of the 

 body. In order to set this question at rest an American 

 geologist, Walcott, conceived the idea of cutting across the 

 bodies of well-preserved trilobites and thus exposing the 

 limbs in section, if they should happen to have left any 

 traces of their presence. He made several thousands of 

 sections, but only some two hundred and seventy of them 

 were found to be of any use. 



Even with these sections it was no easy task to deter- 

 mine the character of the limbs. A single section could 

 show but little ; and it was only by examination of the 

 whole series that ^\'alcott was enabled to form a general 

 idea of the nature of the appendages. 



He found no trace of antennas ; but the head bore four 

 pairs of appendages surroimding the mouth (Fig. 8i. All 

 were jointed, and the iirst three were slender, while the 

 fourth was much stronger and broader. Each segment of 

 the thorax and of the tail also possessed its own pair 

 of limbs ; and these were all of the same cliaracter, but 

 diminished in size towards the tail. Each limb (Fig. 1) 

 consisted of a broad basal joint articulating with the body ; 

 and from this joint sprang two branches, both of them 

 slender and made up of several segments. The outer branch 



Fl&. 3. — Restoration of the under 

 surface of Calymene. (After Walcott.) 



was provided with a row of bristles. Such limbs could 

 not have been used for swimming, and the animal must 

 have crawled upon the bjttom of the sea. 



But this is not aU. 

 Many of the legs also 

 bore a delicate doubly- 

 branched filament, 

 which was often coiled 

 into a spiral. These, 

 no doubt, were the gills 

 or breathing organs. 

 Many living Crustacea 

 have the gills attached 

 to the Umbs, and in the 

 genus called Cyamus 

 these giUs are spiral. 



Walcott's observa- 

 tions thus confirmed the 

 conclusions based on 

 the American specimen 

 of Asaphus already no- 

 ticed, and they were 

 themselves confirmed 

 by the discovery of 

 a second specimen (of 

 a slightly different 

 species) showing the 

 limbs much more per- 

 fectly (Fig. 5). 



So matters remained until the last year or two, when a 



startling discovery 

 was made near 

 Kome, in the State 

 of New York. Tri- 

 lobites are very 

 abundant in certain 

 slaty beds near that 

 town, and they are 

 more perfectly pre- 

 served than any 

 which have hitherto 

 been found else- 

 where. Many of the 

 specimens show the 

 limbs, and some of them even possess antennse, of which 

 no trace had yet been discovered. Most of them belong 

 to the species Triarthms Imki, but other forms are not 

 uncommon. The specimens have been partly described 

 by Matthew and Beecher, and the latter is still engaged 

 in studying them. 



The presence of antennae is, perhaps, the most interesting 

 featm-e of the discovery (Fig. 6). They are attached to 

 the sides of the hypostome on the under-surface of the 

 head, and extend forwards as slender jointed filaments, 

 not unlike the antennic of a lobster. Besides these, the 

 head bears four pairs of appendages, as Walcott had con- 

 cluded. The thorax also and the tail bear Umbs, and all 

 of the appendages except the antenna; are built upon the 

 same plan. Each commences with a basal joint articu- 

 lating with the body of the animal, and from this spring 

 two branches, each of which is made up of a number of 

 joints or segments. 



But although the general plan of all the limbs is the same, 

 the details arc modified in difl'erent parts of the body to 

 suit various purposes. In the head the limbs were required 

 to serve as jaws, while towards the hinder end of the body 

 they were used for swimming. We may look upon the 

 appendages of the second thoracic segment as a kind of 

 middle stage between those extremes. The inner part 



Fig. i. — Section of Cali/meiie. (After 

 Walcott.) A, inner branch of the limb; 

 B, outer branch ; C, spiral gills. 



