BRANCHIOPODA. l ° 7 
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 he branchi- 
opods and the trilobites, and it is only in the Notostraca, with their sessde eyes and 
depressed form, that any comparison can be made. The chie way m which modern Bran- 
chiopoda and Trilobita agree is that both have a vanable number of segments m the body, 
hat number becoming very large in Apus on the one hand and Mesonaas and P.dcunnas 
on the other. In neither are the appendages, except those about the mouth, grouped m 
tagmata Other likenesses are : the Branchiopoda are the only Crustacea other than Trilo- 
bite 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 thev are richly segmented. . . 
In Branchiopoda, the antenna are variable. In the Notostraca they are vestigia , while 
in the males of the Anostraca they are powerful and often complexly developed claspers 
Either condition might develop from the generalized biramous antennae of Trilobita but 
the present evidence indicates a tendency toward obsolescence. Clans' observations indicate 
that the antennae of the Anostraca are developments of the exopodites, rather than of the 
l0 Th d e te 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- 
& 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 fi- 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- rigs. 3, 4) a series of endopodites from the 
pygidium of a voung 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 ot parallelism and does 
not denote relationship. 
