6680 Crustacea. 



live in it. In Crustacea the anterior antennae supply the place of 

 the ears. 



The antennae which cany the acoustic organs surround and 

 enclose the entire apparatus with its bony tissue, leaving, as far as 

 our experience goes, no aperture in the crab, and a very insignificant 

 one, in the lobster ; and, as far as this part is concerned, the 

 apparatus that receives impressions of sound is protected from the 

 influence of external agencies by the stout integumentary tissue, — a 

 circumstance that must preclude it from being at least a very 

 effective organ under ordinary conditions. To obviate this and 

 render it useful to the animal, the external walls are prolonged to a 

 very considerable extent, the greater in comparison with the lessened 

 perfectibility of the internal structure. Upon the slender and length- 

 ened continuation of the integument, intermixed with the numerous 

 more or less strong hairs, are placed the membranous ciliae of which 

 I have spoken. 



These organisms are present, I believe, in all the higher orders of 

 aquatic Crustacea, from the most perfect to the most immature forms, 

 from the youngest to the oldest stage. In the larva of the crab the 

 organ, in the earliest condition, is scarcely observable except by 

 careful examination, by the aid of cautious dissection and a good 

 microscope ; yet it can be clearly defined from any of the other 

 organs by the presence of these delicately-structured ciliae, which 

 assume a size, in relation to the antennae, that gives them a prepon- 

 derating importance. This agrees with observations made on the 

 lower forms. In the sessile-eyed Crustacea the antennae have no 

 internal structure that I can detect that assimilates with the apparatus 

 found in the higher animals, but exists as a member developed to 

 support these membranous ciliae, which appear to me to assume a 

 high importance in their relation to the sense of hearing, particularly 

 in embryonic and lower forms of Crustacea. 



As sound is a minute vibration given by percussion to air, so 

 a parallel result probably is excited by the same means in water, the 

 difference between the two being equal to the difference of the den- 

 sity of the media. Thus, to make the vibration of the more dense 

 material readily perceptible to the consciousness of the animal 

 existing in it, long and delicate organisms, such as these ciliae, must 

 considerably facilitate the power. The term of auditory ciliae 

 appears to be very applicable to them. And in the Amphipoda we 

 have traced what we believe to be a nerve traversing the lower side 

 of the organ to the extremity of the peduncle, and to the root of the 



