ARTHROPOD A — CRUSTACEA — TRILOBITES 293 



species but of differing sizes, just as the record of the growth 

 of a crayfish may be read in the series of molts it discards. 

 During growth, the body elongates through successive molts ; 

 the eyes when present move dorsally toward the glabella, and 

 the free cheeks increase in size. Several molts take place be- 

 fore the introduction of the first thoracic segment, during which 

 the glabella becomes more strongly annulated, the free cheeks 

 appear on the dorsal surface and the introduction of new append- 

 ages and segments to the pygidium produces in that additional 

 grooves in the axial and pleural portions. The pygidium, in some 

 genera, is completed before the introduction of the first thoracic 

 segment. From the anterior edge of the pygidium are devel- 

 oped all thoracic segments and by the forward movement of this 

 growing area the enlargement of the pygidium is effected. 

 Apparently never more than one segment is added at each molt, 

 but there may be more than one molt to each segment. In 

 Triarthrus the full number of body segments was attained when 

 the animal was about 7 mm. long; from then on it merely 

 increased in size until adulthood, with a length of 25-40 mm., 

 was attained. The simplest protaspis is found in the Cambrian 

 rocks ; in later periods it acquires through acceleration in devel- 

 opment adult characters such as dorsal movement of the eyes, 

 free cheeks, etc. 



Relationship. — That trilobites belong to the class Crustacea and 

 are closely related to the sub-class Phyllopoda (e.g. Apus, p. 299), 

 the most primitive of living Crustacea, is shown by the following 

 characters common to both, — (i) variable number of thoracic 

 and abdominal segments among species of each genus ; (2) slight 

 distinction between thoracic and abdominal feet, each of which 

 is more or less leaf-like and respiratory in character, is biramous 

 and usually produced into a gnathobase ; (3) five pairs of head 

 appendages ; (4) an upper lip (hyposf ome) and low^Jip (meta- 

 stome); (5) a_pair of slender, jointed antennules. 



That they are more primitive than the Phyllopoda is indi- 

 cated by the following facts, — (i) all are marine; (2) all seg- 



