426 MR. H. M. BERNARD ON THE CHERNETID^. 



one against another, the lung-books could easily be obtained ; 

 by the development of each chamber into a long iutracellu'jar 

 tubule, the tracheal tuft ; and by the development of one or two 

 chambers into long tubes, the tubular tracheae. 



I therefore offer the suggestion that these ram's-horn invagi- 

 nations opening under the genital operculum, in the Chernetidse, 

 may be the nearest approach to the primitive form of trachea 

 yet discovered. 



In the matter oi tracheae, then, the Chernetidae are very im- 

 portant Arachnids, and throw a strong light on the origin of the 

 whole group from some earlier ancestor, with a pair of limbs and 

 a pair of tracheal invaginations on every trunk-segment. In the 

 Chernetidse all trace of the cephalothoracic tracheae have vanished, 

 unless we can homologize the coxal glands with tracheae. But 

 on the abdomen the first segment has occasionally a very primi- 

 tive form of trachea, the two following segments have normal 

 tuft-tracheae, and the seven following segments seven pairs of 

 vestigial stigmata ! 



The Sensory Organs. 



The structure of the eyes, of which there are in Olisium two 

 pairs, may be seen from the diagram (fig. 15). The lenses were 

 not very compact, the layers of chitin showing, in section, a 

 loose lamination which, if not due to the action of alcohol, must 

 detract from their dioptric efficiency. The retinal cells were very 

 large and seemed to be continued into rods, the distal ends of 

 which were embedded in the pigment-cells forming the cup. I 

 could not find out the connection between the nerve and these 

 inverted retinal cells. 



The cells secreting the lens form a very distinct layer, the 

 vitreous body. This fact is interesting because the lateral eyes 

 of Scorpio, which are apparently the homologues (or ? analogues) 

 of these lateral eyes of Obisium, are said to have no such vitreous 

 layer *. This seems to show how very little value can be 

 attached to the structure of eyes in questions of affinity. 



Perhaps almost as important as the eyes are the sensory 

 organs found in the arthrodial membranes between the thick 

 chitinous ring at the edge* of the coxa and the trochanter on the 

 last two pairs of legs. The fine cuticle seems to project as a 



* Lankester and Eoiirne, " The Minute Structure of the Lateral and the 

 Central Eyes of Scorpio and Limut^ts," Q. J. M. S. xxiii. 



