356 



COMPARATIVE ANATOMY 



CHAP. 



1. Specific Organs of Touch. The points of the limbs, especially 

 of such as serve for locomotion or for holding food, possess a finer 

 sense of touch than the other parts of the surface of the body. 



Special Tactile Setse are the principal organs of touch. These are 

 found chiefly on the antenna?, but also on other extremities, and occa- 

 sionally also on the body itself. These setae are distinguished from 

 other setse, spines, etc., whose function is almost entirely mechanical, 

 by the fact that one or more ganglion cells lie at their bases connected 

 by nerve fibres with the general nervous system (Fig. 239, _D). 



2. Olfactory Organs are found in the shape of pale delicate knobs, 

 filaments, tubes, or points (Fig. 239, A, ), which are often grouped 

 in bundles or transverse rows, and occur on the anterior antennae. 



FIG. 230. A, 7, S, 9, joints of a 13-jointed nagellum of the anterior Antenna of Nebalia (male) 

 with the olfactory tubes. B, Two Olfactory tubes, more strongly magnified. C, Feathered sen- 

 sory seta (auditory hair) from the antepenultimate pair of thoracic limbs of Apseudes, with cuti- 

 cular basal capsule. D, Tactile hair (Hi) of Branchipus. c, Body cuticle ; Jiy, hypodermis cells of 

 the seta ; gz, ganglionic cell ; n, nerve, fibre (after Glaus). 



Less frequently similar structures are also found on the second 

 antennfe. They always occur in greater numbers in the male than in 

 the female. The chitinous cuticle of these olfactory processes is thickest 

 at their bases ; towards their free end it is thin and delicate. At the 

 base of each olfactory process a nerve fibre enters without forming a 

 ganglionic cell, and continues into the interior of the process, running 

 through it and filling it to its free end. The nerve fibres originate in 

 ganglionic cells which, lying in the same or preceding joint of the 

 antenna, belong to the antenna! nerve. 



Whether the so-called Caleeoli of the Amphipoda are olfactory 

 organs, or whether, perhaps, they represent a kind of auditory organ, 

 must remain undecided. 



3. Frontal Sensory Organs. The characteristic position of these 

 organs which occur in one pair is the frontal region very near the 

 brain. They are projecting filaments, cones, rods, or other cuticular 



