42 Mr. C. Spence Bate on the Homologies of the Carapace 



scarcely fused. In Brachyura the whole, more or less, certainly 

 the four first, almost always coalesce, and are generally formed 

 into a very compact mass, so that their position can only be 

 indicated in Sterno7'hynchus by the presence of the olfactory oper- 

 culum. 



In the Macroura this operculum is absent, and an orifice pro- 

 tected by a thin membrane represents the position of the organ. 

 In the Amphipoda the organ is developed in the form of a strong 

 spine or tooth with an orifice at the extremity*. In the Isopoda 

 I have not been able to decipher with confidence the organ of 

 sense in the inferior antenna, but that it exists can scarcely be 

 doubted, since the lower antenna, except in parasitic Crustacea, as 

 the female of Bopi/ms and lona, is I believe never rudimentary. 



The nerve which supplies this organ is a fifth pair or a branch 

 of the fourth. In the Brachyura, in which our dissections have 

 chiefly been made, the fourth pair of nerves extends beyond the 

 inferior antenna, and then terminates in the muscles which raise 

 and close the olfactory operculum. 



This organ, which is described by M. INIilne-Edwards as an 

 organ of hearing, difi"ers in its construction in the different 

 families of Crustacea. In the Brachyura it is a small moveable 

 appendage, situated at the point of articulation between the 

 third and second joints ; it is attached to a long calcareous lever- 

 like tendon, at the extreme limit of which is attached a set of 

 muscles by Avhich it is opened and closed, to assist in which 

 operation at the angle of the operculum most distant from the 

 central line of the animal are fixed two small hinges. When 

 the operculum is raised, the internal surface is found to be per- 

 forated by a small circular opening protected by a thin membrane. 



Among the Macroura this orifice exists at the extremity of a 

 small protuberance, and is not capable of being withdrawn into 

 the cavity of the antenna, as in the Brachyura. 



The next question which we have to consider is, to which sense 

 either of these two sets of organs belongs ; — whether the upper 

 belongs to the auditory and the lower to the olfactory, as I shall 

 endeavour to prove, or vice versa, as maintained by Prof. 

 Milne-Edwards. 



We shall divide the evidences on either side under two heads; 

 the first, that which is derived from an external observation ; and 

 the second, that which is derived from its internal organization. 



First then from external circumstances : An auditory appa- 

 ratus is an organ furnished to an animal for one or both of two 

 objects ; first, for protection from danger ; secondly, for the plea- 



* This will be more cnlargrd ui)ou in the forthcoming Report at the 

 next Meetintc of the Uritish Association. 



