72 RECORD OF CURRENT RESEARCHES RELATING TO 



hirsute than in A, cavaticus, while the broad enveloping form which 

 they take in A. aquaticus readily distinguishes that sjDecies. The 

 number of olfactory organs carried by the antennae never exceeds 

 three in either sex. These are the chief points distinguishing species 

 which are evidently closely allied. 



With regard to the absence of vision, common to this and allied 

 species, this is not always absolute, for two young specimens occurred 

 with eyes possessing pigment-spots ; these were from depths of 200 

 and 300 metres — a remarkable fact, seeing that total obscurity has been 

 shown to exist at 100 metres. To account for this exception, the 

 theory of atavism may be advanced — that the specimens in question 

 were descended from an Asellus which had only recently lost its sight, 

 and that they have reproduced the organs. Cases of heredity are 

 not likely to occur sporadically in such a way as that only two 

 specimens are found occurring side by side with numbers of totally 

 blind but specifically identical forms. Against the hypothesis of 

 heredity in this case must be set the improbability of some few indivi- 

 duals of the species occurring, as these do, under exactly the same 

 unfavourable external conditions as those which have become blind, 

 and yet preserving organs which adrnpt them for seeing ; against which 

 imijrobability the possible shortness of the time of ex^josure to these 

 conditions argues but feebly. 



The sense of hearing has been attributed by Sars to certain pedun- 

 culated hairs borne by the first thoracic limb and the antennae ; they 

 consist of a delicate membrane containing a granular material, and 

 carry a few minute terminal hairs ; those on the legs are carried by 

 its first joint, and are covered by the terga of the segments during 

 walking. De Eougemont's theory that they represent a sixth sense, 

 or one which would transmit impressions of change in the surrounding 

 .medium, appears the most probable one for these abyssal organisms: 

 the position of those on the legs forbids the idea of a tactile function. 



The sense of touch is assigned to the three pairs of hyaline rods * 

 found on the joints of the lesser antennae, and to some hairs on the end 

 of the antennae and caudal appendages. 



Smell is represented by three cylinders inserted into minute, 

 truncate-cone-shaped eminences on the last three joints of the antennae ; 

 they are formed by a membrane with double contour containing 

 granular matter. In A. cavaticus they appear to vary considerably 

 in number, but all authors agree in finding six joints which carry 

 them in one or other of the specimens examined. A. aquaticus has from 

 four to five such organs. In the last two siDCcies they are developed, 

 in point of size, inversely with regard to the presence of visual organs, 

 being small in the species which has eyes. The specimens of A. Forelii 

 which had eyes had the olfactory organs of the same length as in the 

 eye-bearing A. aquaticus. Embryos of A. Forelii are devoid of eyes, but 

 at an early stage agree in all respects with the other two species, and 

 only at a later embryonic stage become distinguishable from H. aqua- 

 ticus by the inferior proportions of the antennae and antennules. 



The interesting question as to the origin of the species is raised 



* Humbert, ' Bull. Soc. Vaud. Sci. Nut ,' xiv. Nos. 75 aud 76. 



