2. Superficially, the eyes of some trilobites do look like those of Limulus, 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 compressed 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.
6. There is nothing among the trilobites comparable to the movable lateral spines of the metasoma of Limulus.
While, as classifications are made up, the Trilobita must be placed in the Crustacea rather than the Arachnida, there is no reason why both the modern Crustacea and the Arachnida should not be derived from the trilobites.
It has been a custom of long standing to compare the trilobite with Limulus. Packard (1872) gave great vitality to the theory of the close affinity of the two when he described the so called trilobite-stage in the development of Limulus polyphemus. His influence on Walcott's ideas (1881) is obvious. Lankester has gone still further, and associated the Trilobita with the Merostomata in the Arachnida.
The absence of antennules at any stage in development allies Limulus so closely with the Arachnida and separates it so far from the Trilobita that in recent years there has been a tendency to give up the attempt to prove a relationship between the merostomes and trilobites, especially since Clarke and Ruedemann, in their extensive study of the Eurypterida, found nothing to indicate the crustacean nature of that group. A new point of view is, however, presented by the curious Sidneyia inexpectans and Emeraldella brocki described by Walcott from the Middle Cambrian.