CLASSIFICATIONS IN NATURAL HISTORY. 217 



nervous system is present and surrounds the oesophagus (30), 

 and expresses the differentiated bilateral symmetry by consist- 

 ing of a double, ventral, gangliated cord (31), and in some gen- 

 era there are differentiated distinct optic organs (32) and special 

 organs of touch (33). The digestive system is differentiated 

 into mouth, sometimes armed with distinct jaws (34) for mas- 

 tication, a distinct oesophagus , a stomach or digestive cavity, 

 an intestine or assimilative canal, and the two openings, mouth 

 and anal, of the digestive canal are permanent. In the An- 

 nelids there is a pseudo-haemal system (35), a vascular dis- 

 tributing system, but not so highly developed as the circulat- 

 ing system of the true Arthropoda. 



In the true Arthropoda, in addition to the elaboration 

 seen in the Vermes, there is differentiated a distinct system 

 of motor organs (36), articulated appendages moved by mus- 

 cle and not by hydrostatic device, and articulated to the 

 segments. The segments are repeated in more definite num- 

 bers, but in the Cambrian there was not a permanent selection 

 as to number. In the Trilobites there was evidently (37) a 

 selection of number of segments in contrast to the indefinite 

 number of the Vermes, which in Eunice gigantea, a modern 

 type, has 400 segments. There was a permanent specializa- 

 tion of a (38) chitinous exo-skeleton, which is a distinct elab- 

 oration of the motor skeletal system, and made possible a 

 number of special differentiations of the motor organs. There 

 was specialization of appendages for special functions ; that is, 

 for sense organs (39), for mouth or mandibular masticatory or- 

 gans (40), for swimming- or locomotion (41), and other sets con- 

 nected with respiratory (42) function. 



The definite differentiation of organs for the respiratory 

 function {gills or branchice) (43) is a further elaboration of the 

 alimentary system, but this function was evidently specialized 

 even in the Cambrian representatives, the Trilobites, which 

 were the most highly elaborated organisms of that era. In 

 these Trilobites we find thus a thorough differentiation of 

 special organs for each of the systems of functions character- 

 istic of the highest type of animals, viz., Correlation, further 

 elaborated in the two systems, (a) motory, illustrated by the 

 muscles and skeletal parts, and (b) nervous system, ganglia, 



