MORPHOLOGY OF TRIARTHRUS 217 



Crustacea, no matter how many nor what kinds of segments 

 may intervene between it and the head. The youngest seg- 

 ment, therefore, is always in the budding zone, just in front 

 of the telson, or terminal somite, and those further anterior 

 and more differentiated are older. This sequential order in 

 the age of the segments and appendages may be greatly 

 obscured in higher forms, so that, as in the Thoracostraca, 

 the last pair of pleopods, forming with the telson the caudal 

 fin, appears at an early stage of the ontogeny. In such 

 cases, as Lang says, "the grade of development and physi- 

 ological importance of a section of the body or of a pair of 

 limbs in the adult animal may be recognized by the earlier 

 or later appearance of their rudiments."* 



In Triarthrus these disturbing factors are hardly to be 

 recognized, for no pair of limbs had an excessive physio- 

 logical importance over any other pair or series of pairs, and 

 increase progressed regularly by the addition of new members 

 in front of the anal segment. The pygidium being formed 

 of fused segments accommodated itself to this kind of growth 

 by pushing forward the series of limbs and by the formation 

 of a new free segment at the posterior end of the thorax. 

 This process of metameric growth continued from the pro- 

 taspis stage with no free thoracic segments, and successively 

 added segment after segment with corresponding moults, 

 until the full complement was reached, after which the 

 moulting resulted mainly in increase in size. The repetition 

 of moults afforded the chief means by which modifications in 

 the appendages could be brought about. 



The earliest protaspis stage shows, from the segmentation 

 of the axis, that there were present five pairs of append- 

 ages on the head and two on the pygidium. 6 The adult 

 animal has thirteen or fourteen free thoracic segments and 

 six pygidial. f Now, so far as is known of trilobite ontogeny, 

 there was never more than one segment added at a single 



* Text-Book of Comparative Anatomy, English edition (Bernard), p. 410. 

 t A few individuals of this species (T. Becki) have been observed with one or 

 two additional thoracic segments. Walcott. 11 



