:276 W. SAYILLE-KENT, CONTRIBUTIONS S TO A 



accompanied by a seta. Tlie significance of these Haller was 

 unable to determine. 



Ke verting now to Limnesia histrionica as personally examined, 

 it has been found easy to recognise two orders of stigmata-like 

 apertures npon the surface of the cuticle those of the one series 

 being accompanied by, and those of the other being devoid of, a 

 setose appendage. In accordance with Haller's observations it has 

 likewise been found that the structures associated with these 

 respective apertures differ to a marked extent from each other, 

 though in a manner distinct from what that authority has 

 described of Hygrohates. In connection with the two pairs of 

 apertures situated towards the anterior region of the dorsal 

 surface are developed the rosette-like clusters of simple pyriform 

 glands first described by Claparede in Atax ci'assijyes and as 

 yet alone recognised in these types. Those belonging to the re- 

 maining order, and numbering upon the dorsal surface as many 

 as seven pairs, are distinguished by the extraordinary length 

 and tenuity of the glandular elements, which consist of a 

 complex network of tubules or cannuli, developed upon a 

 dichotomous plan from the larger common tubular passage 

 that opens upon the dermal apertures. As fully developed in 

 adult examples these glandular tubules of the collecting system 

 so approach one another at the peripheral edges as to form 

 an almost continuous rete beneath the c\iticular surface. This 

 rete, however, does not come in immediate contact with the 

 cuticle, there being interposed between the two that finer net- 

 work of tracheal threads previously referred to as forming a 

 delicate abundantly intercrossing plexus upon the dorsal surface. 

 Prior to arriving at a correct apprehension of the trached- 

 branchial system of Limnesia and other Hydrachnidae it was 

 anticipated that the tubular glandular retia with their external 

 openings might absorb fluid from the exterior and thus form a sort 

 of water-bed from which the superjacent tracheal tubuli could 

 separate out the air requisite for respiratory purposes, and which 

 could not be similarly derived through the relatively dense 

 substance of the cuticle. The discovery, however, of a specialised 

 tracheal-gill rendered nugatory such earlier interpretation and con- 

 tributed towards their identification as specially modified mucous 

 glands. In conformation they are found in part, as in the case 

 of the trachea, to correspond more closely with the homologous 



