V CRUSTACEA— ^EKSOEY ORGANS 357 



appendages, into which 2 nerves, the frontal nerves, enter, generally 

 forming ganglionic cells. In Branchqnif:, in place of the cuticular pro- 

 cess, there is only an inconsiderable thickening of the cuticle with 

 large hypodermis cells surrounded by ganglionic cells lying under it. 

 Frontal sensory organs have been observed not only in the Ento- 

 ■inostraca and Entomostraam larva, but also in the Malacostracan larva 

 (i.e. when it is a Nauplius), and from this we are justified in concluding 

 that they were original structures present in the Crustacean racial 

 form. 



4. Auditory Organs of the Deeapoda. — These lie on the basal 

 joint of the anterior antennae (antennulse). In all Deeapoda they 

 occur as pit- shaped depressions of the chitinous integument, which 

 generally remain open, but in a few cases (Hippolyte) may close and 

 form a vesicle. In the open auditory vesicles the aperture is often 

 covered by a compact row of setae projecting from one of the edges, 

 less frequently by a thin projecting fold. The auditory pits contain 

 sand particles taken in from outside, which probably function as 

 otoliths, like the concretions of fluorcalcium in the closed auditory 

 vesicles. At the base of the auditory pits, or on the inner wall 

 of the auditory vesicles, feathered hairs arise; these are (1) otolith 

 hairs, which carry the otoliths, and (2) free hairs, projecting 

 into the cavity of the auditory pit. The distinctly marked, swollen 

 base of the auditory hair is extremely delicate and thin-walled, 

 and permits a considerable oscillation of the hair in response to the 

 waves of sound. The auditory nerve, which branches from the 

 antennal nerve and has its root in the brain, first sends ofl^ fibres to 

 single ganglionic cells. Filaments from these ganglion cells enter the 

 auditory hairs at their bases and are attached near their points to rod- 

 shaped bodies. 



It is usually said of the two closed auditory vesicles of the Mysidce {Sehizo- 

 poda), that they Ue in the tail. More correctly, they lie in the inner lamella (endo- 

 podite) of the last pair of pleopoda, which, together with the telson, form the caudal 

 fin. Their structure does not deviate essentially from that of the Decapodan 

 auditory organs. They are innervated from the last abdominal ganglion. 



03:ycej)halus {AmpMpoda) possesses two auditory vesicles lying above the brain 

 and containing otoliths. 



There is some justification for classing the sensory organs of the Deeapoda, Schizo- 

 poda, and Amphipoda above described as auditory organs ; but we must not fail to 

 mention that more recent experimental investigations greatly support the view that 

 they also serve for regulating the position of the body, and for maintaining its 

 equilibrium. 



Feathered setEe, which in structure show great agreement with the auditory hairs 

 of the Deeapoda, occur on the antennse and also in other parts of the body in many 

 Malacostraca, and have often been considered as auditory organs. This is, however, 

 still an undecided point. It is, however, probable that the auditory organs of the 

 Deeapoda have developed phylogenetically by the localisation of feathered sensory 

 hairs and by the pit-like depression of those parts of the integument which carried 

 them. The utilisation of foreign particles of sand as otoliths also supports this view. 



