GENERAL SUMMARY. 213 



tions. In the scorpion the genital papillae and the pectines illustrate the con- 

 version of entire appendages into complex sense organs. The taste organs and 

 slime buds increase by division; the former may thus give rise to numerous 

 sensory cells arranged in long, straight lines. 



The segmental taste organs and slime buds of arachnids are the forerunners 

 of the special cutaneous organs of vertebrates; the taste organs of the arachnids 

 corresponding with the taste buds of the vertebrates, and the slime buds probably 

 in part with the neuromasts or lateral line organs. The fields of taste organs 

 and slime buds of the arachnids are represented in vertebrates by the placodes 

 which initiate a line of taste organs, or of neuromasts. The structure of the organs, 

 the number and location of the principal lines of organs in the embryo, their 

 relation to peripheral nerves, and to the tracts in the brain, are in the main very 

 similar in both vertebrates and arachnids. (Figs. 58 and 65, g.n.r.) 



In Limulus the enormous integumentary nerve of the cheliceral segment, Fig. 

 70, deserves special attention. It resembles the ramus lateralis accessorius, 

 which in ganoids and bony fishes is distributed to the back, tail, and fins, wher- 

 ever taste buds are found (Johnston). Unfortunately the character of the 

 terminals to this remarkable nerve in Limulus was not determined; but it is un- 

 questionably a purely sensory nerve, supplying the neural surface of the entire 

 posterior part of the body. Its ramification is specially rich around the 

 entrance to the gill chamber. 



1 6. The general cutaneous sense organs in Limulus are scattered over all 

 parts of the body. They represent various minor modifications of the slime buds, 

 taste buds, and of free nerve endings, and serve either as tactile, temperature, or 

 chemotactic organs. They are connected with a loose subdermal nerve plexus 

 which, in the thoracic and abdominal shields, is derived from the ramifications 

 of the haemal nerves; that on the surface of the gills, operculum, and terminal 

 joints of the leg is derived from the ramifications of the neural nerves. 



The central terminals of the general cutaneous components of the haemal 

 nerves end in a large tract on the median side of each crus, haemal to the gustatory 

 tract. (Fig. 56, G.c.tr.) 



17. The peripheral nervous system of arachnids attains a condition similar 

 to that in vertebrates. There are two main systems of mixed nerves, the neural 

 and haemal. The neural nerves have enormous ganglia which develop independ- 

 ently of the neural axis; the haemal nerves are without ganglia. Both sets remain 

 separate in the anterior head region, but in the posterior head region, and in the 

 trunk, they may unite to form nerves of the spinal cord type, that is, single nerves 

 with separate roots, ganglionated neural ones and non-ganglionated haemal ones. 



1 8. Further specialization takes place through the separation from the primi- 

 tive segmental nerves of those components that have similar peripheral and central 

 terminals, and their union to form a new system of nerves with a common central 



