272 AKTHROPODA [CH. 



than the neighbouring ectodermal cells, and this thicker patch of 

 cuticle forms the lens. Some of the pit-cells secrete visual rods 

 on their lateral surfaces. Several of these rods coalesce to form a 

 characteristic rhabdome, or striated spindle characteristic of 

 Arthropodan eyes. The bases of the cells which secrete the rods are 

 prolonged into nerve fibres. The central eyes have a layer of clear 

 cells lying above the visual cells ; this is the crystalline layer and 

 acts the part of an additional lens. The visual cells of the central 

 eye are arranged in groups called retinulae, consisting of six or 

 seven cells which surround a. central rhabdome. 



Scorpions usually hide under rocks and stones during the day, 

 being often very intolerant of heat, but they cfeep out as dusk 

 comes on and run actively about. The Scorpion is viviparous, the 

 young being born in a condition resembling their parents. 



Order V. Xiphosura. 



A very peculiar aquatic Arachnid called Limulus, or popularly 

 the " King-crab/' inhabits the warm seas on the Western side of the 

 Pacific Ocean and along the shores of the "Western Atlantic. It is 

 a littoral form, that is to say, it lives not far from the shore ; it 

 burrows in sand or mud at a depth of from two to six fathoms, often 

 lying with only its eyes, which are on the top of the body, exposed. 



The shape of the body is something like a half-sphere with a 

 piece cut out and a long spine is attached to the truncated side. 

 This spine has given the name Xiphosura (Or. &'<os, a sword ; ovpd, 

 a tail) to the Order. The half-sphere is hinged, and the part in 

 front of the hinge is the prosoma; the rest is the abdomen, or 

 meso- and meta-soma. On the upper surface of the half- sphere are 

 a pair of simple eyes near the middle line, and there are a pair of 

 compound eyes situated further back nearer the edge. The under 

 surface of the half-sphere is partially hollowed out and concealed in 

 this hollow on each side of the middle line of the prosoma are six 

 pairs of appendages. The most anterior of these are typical nipper- 

 like chelicerae, the next is not specially modified to form a pedipalp, 

 but it and the remaining four pairs are walking legs. All of them 

 send inwards a spiny gnathobase, which helps to form the border 

 of the mouth. The sixth pair of limbs end in some flattened blade- 

 like structures which assist in digging and burrowing in the sand 

 and in extracting the worms which form the principal item of the 

 diet of the King-crab. Behind these limbs are a pair of oval spiny 



