218 STUDIES IN EVOLUTION 



moult, though there is no evidence that there may not have 

 been more moults than segments between the protaspis stage 

 and the finished segmentation. In Triarthrus the average 

 full number of segments was attained by the time the animal 

 reached a length of about 7 mm. So that the limbs of the 

 anterior thoracic segment in an individual 7 mm. in length, 

 and containing the full complement of fourteen free and six 

 pygidial segments, must have undergone at least seventeen 

 moults. The second thoracic segment, therefore, at this 

 stage of growth would have been moulted sixteen times, the 

 fifth thirteen times, the tenth eight times, and the fourteenth 

 four times. The length of full-grown individuals is from 

 25 to 40 mm., and to have reached this size a considerable 

 number of additional moults must have occurred in which all 

 the segments participated alike. 



Some mention should be made of the probable method of 

 respiration of Triarthrus. No traces of any special organs 

 for this purpose have been found in this genus, and their 

 former existence is very doubtful, especially in view of the 

 perfection of details preserved in various parts of the animal. 



The delicacy of the appendages and ventral membrane of 

 trilobites and their rarity of preservation are sufficient demon- 

 stration that these portions of the outer integument were of 

 extreme thinness, and therefore perfectly capable of perform- 

 ing the function of respiration. Similar conditions occur in 

 most of the Ostracoda and Copepoda, and also in many of 

 the Cladocera and Cirrepedia, where no special respiratory 

 organs are developed. 



The fringes on the exopodites in Triarthrus and Trinucleus 

 are made up of narrow, oblique, lamellar elements becoming 

 filiform at the ends. Thus they presented a large surface 

 to the external medium, and partook of the nature of gills. 

 But, as Gegenbaur says, "the functions of respiration and 

 of locomotion are often so closely united that it is difficult to 

 say whether certain forms of these appendages should be 

 regarded as gills, or feet, or both combined." * For purposes 



* Elements of Comparative Anatomy,, English edition (Bell and Lankester), 

 p. 241. 



