THE EYES OF SCORPIONS 131 



expresses the opinion that the structure cannot be regarded as an eye in 

 the strict sense of the word, and prefers to speak of it as simply a " pig- 

 mented sense-organ." Whether this remarkable organ is a vestigial eye 

 or belongs to any other type of sense-organ, its presence in the centre of the 

 dorsal shield in the median plane between the two pairs of laterally placed 

 eyes is of very considerable importance, and it would be interesting to learn 

 if further research reveals the presence of such an organ in related species. 



It may be noted here that in the Linguatulida or Pentastomida, which 

 are degenerate and parasitic Arachnida, the nervous system is greatly 

 reduced and the organs of special sense have entirely disappeared. One 

 species, Pentastomum tcenioides, has been found in the frontal air sinuses 

 and maxillary antra of the dog and wolf. Its embryos, escaping and 

 falling on grass or other herbage, are eaten by hares and rabbits, perforate 

 the w^lls of the stomach and intestine, become encysted in the liver, and 

 undergo a metamorphosis. If the hare or rabbit should be eaten by a dog, 

 the young pentastoma may find their way into its frontal air sinuses or antra. 



In the Tardigrada or " bear animalcules," which are soft-skinned 

 animals about 1 mm. in length living on damp moss or in fresh or salt 

 water, there is also a reduction of the central nervous system and the sense- 

 organs are represented merely by a couple of eye-spots at the anterior end 

 of the body. 



The Eyes of Scorpions 



These have been studied by Lankester and Bourne (1883) and J. S. 

 Kingsley (1886). The lateral eyes are placed on the margin of the proso- 

 matic shield in a group on each side (Fig. 93). The number of separate 

 lenses differs in various subgenera, and each lens indicates a separate eye. 

 In Androctonus the eyes are more numerous than in other scorpions, each 

 lateral group in A . fanestus showing as many as five lenses — three larger 

 and two smaller. 



The lenses are of the cuticular or corneal type, and are highly convex 

 on the inner or deep surface (Fig. 94). The hypoderm cells beneath the 

 surrounding cuticle are continued on to the circumferential aspect of the 

 cuticular lens, where they form the vitreous " marginal body " or the 

 " perineural epidermic cells " ; whereas beneath the central part of the 

 lens the hypoderm cells are specially modified to form the retina. This 

 consists of elongated sensory cells the inner ends of which are continuous 

 with the fibres of the optic nerve. In the body of each sensory cell deep 

 to the large spherical nucleus is a refringent globule, called the post- 

 nuclear phgeosphere, and in relation with the outer ends of the sensory 

 cells are elliptical refractile elements termed rhabdomes. Interneural 

 epidermic cells and some cells of a connective tissue type are also present. 



