350 CRUSTACEA. 



of a Burnet Moth is represented by a hind wing appears to show that it 

 is not always so confined. In view of such a case can it be assumed that 

 a lobe of the head not originally a limb, but lying in the line of the limbs, 

 cannot take on the character of a limb ? The amount of weight which 

 we allow to the results of Herbst's experiments in their bearing on the 

 appendicular nature of the paired eyes depends on the answer to this 

 question. 



Vesicular^sensg. organs, usually opening to the exterior and 

 containing delicate sensory hairs and hard particles, are found 

 in the basal segment of the first antenna in many Decapods, 

 and closed vesicles of similar structure occur in the uropods of 

 the Mysidae. The open sacks contain sand, which is renewed 

 at each ecdysis. The closed vesicles are said to contain con- 

 cretions of calcium fluoride. It has been generally assumed 

 that these are auditory organs ; * but Kreidl f has shown, 

 by an ingenious experiment, that they have another function. 

 Specimens of the prawns Palaemon xiphius and P. squilla, 

 shortly before an ecdysis, were kept in an aquarium in which 

 sand at the bottom was replaced by iron in a state of fine 

 subdivision. At the ecdysis the lining of the sack of the 

 sense organ was shed, with the rest of the outer cuticle, and the 

 sack was furnished afresh by the Palaemon from the materials 

 available, namely iron particles. On bringing a magnet 

 into its neighbourhood the Palaemon was now found to 

 incline the body, so that the median plane was directed 

 obliquely according as the particles were attracted to one side 

 or the other of the sack, and gave rise to a corresponding 

 stimulus to the hairs in it. It was thus apparent that a 

 function of the sack was to inform the animal, normally by the 

 action of gravity on the contained particles, of its relations 

 in space. DelageJ has come to similar conclusions on the 

 function of the organs in the tail of Mysidae. 



Beer concludes that all the organs, generally regarded until 

 lately as otocysts in the Crustacea, are in reality of similar 

 function, i.e. that they are not auditory but, to use Verworn's 

 name, statocysts. It would appear however, from the possession 



* Cf. Hensen, Studien iib. d. Gehororgan der Decapoden, Zeit. /. wis. 

 Zool. Bd. 13, 1863. 



t Physiologie der Ohr-labyrinth, Sitz. Ber. Akad. Wien., Bd. 102, 

 Abth. 3, p. 149. 



J Comptes Rendus, T. 103 (1886). 



Statocysten-function, Arch. f. ges. Physiologie, Bd. 73 (1898). 



