C. E. Beecher—The Morphology of Triarthrus. 195 



navigation. The form of the animal and the multiplicity of 

 locomotor organs were well adapted for rapid motion either along 

 the sea-bottom or through the water. 



The youngest and most immature limbs are on the pygidium, and 

 in a young trilobite they are very much like those in the larval 

 Apus (4) and are typically phyllopodiform. According to the law of 

 morphogenesis, these limbs may be taken as of pliylogeuetic value 

 and indicative of the primitive type of limb structure. 



The whole series of endopodites anterior to the last two or three 

 show modifications from the phyllopodous type, the change involving 

 progressively from one to all of the endites. The endopodites of 

 the pygidium have a true phyllopodiform structure, and are com- 

 posed of broad leaf-like joints, wider than long. This character is 

 gradually lost in passing anteriorly, the distal endites being the ones 

 first affected. By the time the anterior pygidial limb is reached, the 

 three distal joints are longitudinally cylindrical. The ninth thoracic 

 endopodite shows a fourth endite becoming cylindrical, and on the 

 first and second thoracic legs even the proximal ones are thus 

 modified, making all the endites of these limbs slender in form. 



This gradual modification of a phyllopodiform swimming member 

 into a long, jointed, cylindrical crawling leg deserves more than 

 passing notice, for here, probably, better than in any known recent 

 form, can the process and its significance be studied. No living type 

 of crustacean more nearly conforms to the theoretical archetype of 

 the class than do the trilobites ; and as Triarthrus belongs to an 

 ancient Cambrian family, it may be expected to retain very primitive 

 characters. 



In this genus several causes evidently influenced the modification 

 of the appendages. First may be mentioned the specialization 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 gradually 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 compared 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 segment, 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 



