136 THE PINEAL ORGAN 



in these. Thus, the likeness of the larval form of Limulus to the Trilobites 

 is so close that the resemblance was regarded as unquestionable evidence 

 of the origin of Limulus from a Trilobite stock. The resemblances are 

 chiefly in the anterior part of the body, and concern the structure and 

 general form of the head-shield and the relative position of the sense- 

 organs, including the median and lateral paired eyes ; on the other hand, 

 the contrast between the appendages of the prosoma and those of the 

 abdomen which is so marked a feature in Limulus, is absent in the Trilobite, 

 and Limulus shows no trace of the antennae of the Trilobite. Thus, 

 according to MacBride, the so-called trilobite larva of Limulus represents 

 not a Trilobite but an Arachnid, which is not very unlike the adult animal, 

 but with segments which are free from one another and without the long 

 tail. Such Arachnids are known to have existed in the Silurian epoch 

 (Hemiaspis), but the stock from which the Arachnids and Trilobites 

 diverged must have been still farther back. The conclusion to be drawn 

 from these premises is that the head region, with its important sense- 

 organs, although it has undergone adaptational changes, has remained in 

 its general form essentially the same since the early palaeozoic periods ; 

 and it was thought that the Protostraca, a name suggested by Korschelt and 

 Heider for the precursors of the Palaeostraca, were the ancestors of the 

 Arachnids, Trilobites, and possibly also of the Ostracodermata, or ancient 

 mailed-fishes ; further, that they possessed sense-organs, visual and static 

 situated in the head-shield in the same relative positions to one another 

 as in Apus, Limulus, certain insects, spiders, and vertebrates. 



The Eyes of Trilobites 



The existence of eyes has been demonstrated in most of the Trilobites, 

 and they present great variations in form and size. By far the greater 

 number are of the composite, faceted type ; they are placed laterally, 

 and are supported by the movable cheeks ; the visual area is usually 

 crescentic, but it may be rounded or oval in form. The adjoining part 

 of the fixed cheek is often raised in the form of a lip or palpebral lobe. 

 Their size varies ; they may be quite small, as in Encrinurus and Trimero- 

 cephalus, or very large as in A^glina (Fig. 97, A), in some species of which, 

 nearly the entire area of the movable cheeks is faceted and the visual 

 surface extends right round the outer edge of the head-shield. In one 

 species, Remopleurides radians, the number of facets has been estimated 

 to be 15,000. In some Trilobites the eyes appear to be absent altogether, 

 e.g. Conocoryphe and Agnostus, and in others they are so difficult to detect 

 that for a long time they remained unrecognized — Agraulus, Sao, Ellipso- 

 cephalus. 



The paired lateral or composite eyes of Trilobites are of two types : 



