254 C. E. JBeecher — Morphology of Triarthrus. 



zation into oral organs of the gnathobases of the head, which 

 would tend toward a reduction of the other portions of the 

 limbs. Next, the assumption of a walking habit would gradu- 

 ally lead to a corresponding adaptation of the anterior thoracic 

 endopodites, this region of the body being naturally the place 

 where they would be most operative. Lastly, any tendency to 

 change the form of the anterior limbs would be accelerated 

 through the greater number of moults they undergo as com- 

 pared with the abdominal appendages. 



Since the anal segment of Crustacea contains the formative 

 elements out of which all the trunk segments are successively 

 developed, it may be considered as the same segment in all 

 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 physiological 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 physiological 

 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 push- 

 ing 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 protaspis 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 appendages on 

 the head and two on the pygidium. 8 The adult animal has 

 thirteen or fourteen free thoracic segments and six pygidial.f 



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

 f A. few individuals of this species (1. Becki) have been observed with one or 

 two additional thoracic segments. . "Walcott. 11 



