260 



COMPARATIVE ANATOMY. 



There is yet another system of nerve-branches in connection with 

 this plexus ; it is principally applied to the large branches of the 

 tracheae, and the muscles of the stigmata. This arrangement is 

 brought about by a nerve-filament, which runs on the surface of the 

 ventral chain, and is divided into two fork-like branches in front of 

 each ganglion (Nervi transversi accessorii). The branches receive 

 nerve-twigs from the upper chord of the ventral chain, and pass 

 partly outwards to the branches of the trachea) and tho muscles of 

 the stigmata, and partly backwards, whore they unite in the middle 

 line, and at the next ganglion repeat this arrangement. 



Sensory Organs, 



Tactile Organs. 



§ 201. 



The sensory organs of the Arthropoda are, for the most part, 

 allied to those of the Vermes. All but a few indicate a connection of 



this kind, and these few are to be re- 

 garded as arrangements which are 

 developed in this division only. Tho 

 carapace-like covering of the body 

 of most Arthropoda requires special 

 organs to produce the sensation of 

 touch ; the form- elements of these are 

 connected with ganglionic cells, and 

 form rod-like nerve-endings. These 

 ganglionic cells are generally struc- 

 tures which are derived from the ecto- 

 derm, and the whole apparatus not 

 unfrequently retains its primitive 

 position. 



These end organs, which are found 

 in the most different parts of the body, 

 form indifferent sensory organs, which 

 in certain parts take on the form of 

 tactile organs (cf. Fig. 133). Organs 

 of this kind are mostly found on the 

 appendages, where they present rod- 

 shaped projecting ends. 



In the division of the Crustacea 

 these tactile rods have been recog- 

 nised in many forms, and that not 

 only on the antennae, especially in the lower Crustacea, but even on 

 other appendages of the body. In the Myriapoda and Insecta 

 there are tactile rods on the antenna?, and in the latter they are 

 also found on the tarsal joints of the feet. 



Fig. 133. Nerve - ending with 

 tactile rods, from the proboscis 

 of a Fly (Musoa). n Nerve. 

 g Ganglionic swelling. s Tactile 

 rods, c Fine hairs of the cuticle 

 (after Leydig). 



