TYPE ARACHNID A. 465 



jointed palps; the second pair, wanting in the females of 

 some species, but always present in the males, arise from the 

 ventral surface of the segment, and are curved jointed struc- 

 tures serving to carry the ova ; while the third pair are ex- 

 ceeding long jointed walking-legs. The next three segments 

 also bear long walking-legs, the last o'ne having attached to 

 it the usually unsegmeuted rudimentary abdomen. 



The body and the appendages are encased in a well-de- 

 veloped chitinous cuticle, and there are no indications of 

 special respiratory organs. The heart lies immediately be- 

 neath the dorsal integument and is a simple tubular organ 

 with from two to three pairs of ostia. 



The portion of the digestive tract which lies within the 

 proboscis is lined with chitin and opens behind into an 

 elongated mid-gut, from which long diverticula extend out into 

 the chelicerse and the proboscis and into the walking-legs, 

 sometimes reaching even into the terminal joints of the latter. 

 A short hind-gut leads to the anus at the tip of the abdomen. 



The nervous system consists of a supracesophageal gan- 

 glionic mass, from which arise the optic nerves and those for 

 the chelicerse, as well as certain nerves passing to the pro- 

 boscis. Connected with this brain by circumoesophageal com- 

 missures is a ventral chain consisting of five pairs of ganglia, 

 the first pair of which is really formed by the fusion of two 

 pairs, distinct in the embryo, and innervates the palps and the 

 ovigerous legs, while the four pairs of walking-legs are sup- 

 plied by the remaining four pairs. Finally one or two small 

 ganglia also occur, innervating the abdomen. The eyes are 

 four in number, situated at equal intervals upon a small 

 domelike elevation on the dorsum of the first thoracic seg- 

 ment, which, it is to be remembered, is compound. Each eye 

 is covered by cuticle, sometimes thickened so as to form a 

 lens, below which is a layer of cells forming the corneal or 

 cuticular hypodermis. Below this comes a thick layer com- 

 posed of retinal elements with nuclei in their outer portions 

 and rodlike bodies towards the inner ends where the} r rest 

 upon a layer of pigment. 



These eyes recall the postbacillar eyes of the Arachnids by their struct- 

 ure, but show one remarkable peculiarity, i.e., a distinctly bilateral ar- 



