BRANCHIOPODA. 



of most writers on the subject. Fundamentally, a great deal of the argument seems to be 

 that Apus lies the nearest of any modern representative of the class to the theoretical crus- 

 tacean ancestor, and as the trilobites are the oldest Crustacea, they must be closely related. 

 Most writers state that the trilobites could not be derived from the Branchiopoda (see, how- 

 ever, Walcott 1912 A), nor the latter from any known trilobite, but both subclasses are be- 

 lieved to be close to the parent stem. 



Viewed from the dorsal side, there is very little similarity between any of the branchi- 

 opods and the trilobites, and it is only in the Notostraca, with their sessile eyes and 

 depressed form, that any comparison can be made. The chief way in which modern Bran- 

 chiopoda and Trilobita agree is that both have a variable number of segments in the body, 

 that number becoming very large in Apus on the one hand and Mesonacis and Padeumias 

 on the other. In neither are the appendages, except those about the mouth, grouped in 

 tagmata. Other likenesses are : the Branchiopoda are the only Crustacea, other than Trilo- 

 bita, in which gnathobases are found on limbs far removed from the mouth ; the trunk limbs 

 are essentially leaf-like in both, though the limb of the branchiopod is not so primitive as 

 that of the trilobite; caudal cerci occur in both groups. 



If the appendages be compared in a little more detail, the differences prove more strik- 

 ing than the likenesses. 



In the Branchiopoda, the antennules are either not segmented or only obscurely so. In 

 trilobites they are richly segmented. 



In Branchiopoda, the antennae are variable. In the Notostraca they are vestigial, while 

 in the males of the Anostraca they are powerful and often complexly developed claspers. 

 Either condition might develop from the generalized biramous antennas of Trilobita, but 

 the present evidence indicates a tendency toward obsolescence. Claus' observations indicate 

 that the antennae of the Anostraca are developments of the exopodites, rather than of the 

 endopodites. 



The mandibles and maxillae of the Branchiopoda are greatly reduced, and grouped 

 closely about the mouth. Only the coxopodites of the Trilobita are modified as oral appen- 

 dages. 



The trunk limbs of Apus are supposed to be the most primitive among the Branchio- 

 poda, and comparison will be made with them. Each appendage consists of a flattened axial 

 portion, from the inner margin of which spring six endites, and from the outer, two large 

 flat exites (see fig. 34). This limb is not articulated with the ventral membrane, but attached 

 to it, and, if Lankester's interpretation of the origin of schizopodal limbs be correct, then 

 the limb of Apus bears very little relation to that of the Trilobita. In Apus there is no distinct 

 coxopodite and the endobases which so greatly resemble the similar organs in the Trilobita 

 are not really homologous with them, but are developments of the first endite. Beecher's 

 comparison of the posterior thoracic and pygidial limbs of Triarthrus with those of Apus 

 can not be sustained. Neither Triarthrus nor any other trilobite shows any trace of phyl- 

 lopodan limbs. Beecher figured (1894 B, pi. 7, figs. 3, 4) a series of endopodites from the 

 pygidium of a young Triarthrus beside a series of limbs from a larval Apus. Superficially, 

 they are strikingly alike, but while the endopodites of Triarthrus are segmented, the limbs of 

 Apus are not, and the parts which appear to be similar are really not homologous. The 

 similarity of the thoracic limbs in the two groups is therefore a case of parallelism and does 

 not denote relationship. 



