Il8 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



Sixthly, there are frequently lateral spines on the pleura as in Limiilus. No crustacean 

 has lateral pleural spines. 



These points may be taken up in order. 



1. If trilobites have one appendage-bearing segment in front of the mouth, they are 

 Arachnida; if two, Crustacea. This is based on the idea that in the course of evolution 

 of the Arthropoda, the mouth has shifted backward from a terminal position, and that as a 

 pair of appendages is passed, they lose their function as mouth-parts and eventually become 

 simple tactile organs. Thus arise the cheliceras of most arachnids, and the two pairs of 

 tactile antennae of most Crustacea. This theory is excellent, and the rule holds well for 

 modern forms, but as shown by the varying length of the hypostoma in different trilobites, 

 the position of the mouth had not become fixed in that group. In some trilobites, like Triar- 

 thrus, the gnathobases of the second pair of appendages still function, but in all, so far as 

 known, the mouth was back of the points of attachment of at least two pairs of appendages, 

 and in some at least, back of the points of attachment of four pairs. As pointed out in the 

 case of Calymene and Cerdurus, the trilobites show a tendency toward the degeneration of 

 the first and second pairs of biramous appendages, particularly of the gnathobases. They 

 are in just that stage of the backward movement of the mouth when the function of the 

 antennae as mandibles has not yet been lost. If the presence of functional gnathobases back 

 of the mouth, rather than the points of attachment in front of the mouth, is to be the guide, 

 then Triarthms might be classed as an arachnid and Calymene and Isotelus as crustaceans. 

 In other words, the rule breaks down in this primitive group. 



2. Superficially, the eyes of some trilobites do look like those of Limulns, but how 

 close the similarity really was it is impossible to say. The schizochroal eyes were certainly 

 very different, and Watase and Exner both found the structure of the eye of the trilobite 

 unlike that of Limulus. 



3. The importance of the trilobate form of the trilobite is very much overestimated. 

 It and the pygidium are due solely to functional requirements. The axial lobe contained 

 practically all the vital organs and the side lobes were mechanical in origin and secondarily 

 protective. That the crustacean is not trilobate is frequently asserted by zoologists, yet 

 every text-book contains a picture of a segment of a lobster with its axial and pleural lobes. 

 It is a fundamental structure among the Crustacea, obscured because most of them are com- 

 pressed rather than depressed. 



4. The pygidium of trilobites is compared with the metasomatic shield of Limulus. No 

 homology, if homology is intended, could be more erroneous. The metasomatic shield of 

 Limulus is, as shown by ontogeny and phylogeny, formed by the fusion of segments formerly 

 free, and includes the segments between the cephalic and anal shields, or what would be 

 known as the thorax of a trilobite. No trilobite has a metasomatic shield. The pygidium 

 of a trilobite, as shown by ontogeny, is built up by growth in front of the anal region, and 

 since the segments were never free, it can not strictly be said to be composed of fused 

 segments. Some Crustacea do form a pygidial shield, as in certain orders of the Isopoda. 



5. The post-anal spine of Dalmanites and some other trilobites is similar to that of 

 Limulus, but this seems a point of no especial significance. That a similar spine has not 

 been developed in the Crustacea is probably due to the fact that they do not have the broad 

 depressed shape which makes it so difficult for a Limulus to right itself when once turned 

 on its back. Relatively few trilobites have it, and it is probably correlated with some special 

 adaptation. 



