MEDIAN EYES IN INVERTEBRATES 



349 



name " Palaeostraca " being reserved for the whole group of known fossils 

 which includes such primitive types of marine arthropods as the Giganto- 

 straca, including Eurypterus, Pterygotus (Fig. 96, Chap. 11, p. 135), and 

 Stylonurus, sometimes spoken of as the sea scorpions, and also the Xipho- 

 sura or Merostomata, of which Limulus, the king crab, is a living example 

 (Fig. 69, Chap. 11, p. 108). 



The trilobites have affinities with both the Crustacea and the arachnids, 

 or spiders, and are usually allocated to a separate subclass or are classified 

 as an appendix of the Crustacea or Arachnida, the trilobites being regarded 

 as a precursor of both. The trilobites and all the marine fossil arthropods 



A — Section through the parietal eye vesicle of a scorpion (stage H), showing 

 the approaching pallial folds previous to the union of the two retinas. 



B — Section through the parietal eye of a newly born scorpion showing the parietal 

 eye vesicles and the ventricle, V., bounded by the optic ganglia, the pallial 

 folds and the neuromeres of the fore-brain. The ventricle extends down- 

 ward and forward into the cavities of the olfactory lobes, ol. v. 



mentioned above, include types which either in the adult or larval form 

 show indications of (1) both median and lateral eyes, (2) only median 

 eyes, (3) only lateral eyes. In some, known as " blind trilobites," eyes 

 are said to be absent. It is probable, however, that in many adult speci- 

 mens which have been described as blind, median eyes may have been 

 present in the larval form which disappeared and left no traces of their 

 existence in the adult animal. The lateral eyes of trilobites are usually 

 faceted or aggregate eyes. These are borne on the movable or lateral 

 cheeks and, as have been described on p. 137, are of two types : (1) holo- 

 chroal, in which the visual area is covered by a smooth, continuous film 



