C. E. Beecher — Morphology of Triarthrus. 255 



Now, so far as is known of trilobite ontogeny, there was 

 never more than one segment added at a single 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 seg- 

 ments 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 comple- 

 ment of fourteen free and six pygidial segments, must have 

 undergone at least seventeen moults. The second thoracic seg- 

 ment, 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 4:0 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 Ti'iarthrus. .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 G-egenbaur says, " the functions of respiration and of loco- 

 motion 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 of loco- 

 motion, the limbs of the cephalon and pygidium were of feeble 

 assistance compared with those on the thorax, and in the higher 

 Crustacea, these two regions are the ones where the greatest 

 branchial specialization takes place- 

 Tale Museum, New Haven, Ct, February 24th, 1896. 



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

 p. 241. 



