158 Proceedings of the Roijcd Physical Society. 



closed, but that their lumen communicates freely with the 

 canal over which they stand ; this seems to indicate that the 

 closure of these fringing setse is of comparatively recent ap- 

 pearance among the Decapods, supporting the views of Boas/ 

 who, while he separates Thysano'pocla and the other Euphau- 

 sidse from the Mysidee and Lophogastridse, and regards them 

 as the most primitive forms of the Malacostraca, still con- 

 siders that they are not very far distant from the Decapoda 

 natantia, to which again Astacus is one of the most nearly 

 allied forms among the Deca'poda reptantia. Beyond the 

 point of closure, where they are nearly cylindrical, these setae 

 are fiat and band-like, so that in a cross section they have 

 the form of an elongated rectangle with the corners rounded 

 off. From the anterior and posterior flat surfaces no bristles 

 arise, but on each of the two edges there is a single row of 

 very delicate secondary bristles, which are solid, and the 

 longest of which are about 0'2 millimetres in length, and 

 between O'OOl and 0-002 millimetres broad at the base. The 

 wall of the primary seta, except at the base where it is thick 

 and strong, is much thinner than that of the tactile setae, 

 and on the edges has a curved outline, the depressions in 

 which correspond to the points of insertion of the bristles, 

 while the side towards the lumen has projections which cor- 

 respond to these depressions {see PL IX., Fig. 13). The 

 lumen is filled with fluid, and may contain a few granules. 

 The two rows of bristles do not correspond in insertion on 

 the opposite edges of the setae. The joint about the middle 

 of the seta, which is so marked in the tactile form, is liere 

 less noticeable, as it is masked by the insertion of the bristles ; 

 it can, however, be made out. It is to be noticed that the 

 flat surfaces of these setae always correspond to the surfaces 

 of the appendage or part of the segment to which they are 

 attached ; thus, in the swimmerets, where the surfaces of the 

 appendage are anterior and posterior, those of the setae are 

 also anterior and posterior ; on the pleura of the abdominal 

 somites, where the surfaces are right and left, or internal 

 and external, the surfaces of the setae again correspond. 



1 Studien iiber die Verwandtscliaftsbezieliungen der Malakostraken, Mor- 

 pliolog., Jahrbuch 8, 1883, pp. 521 et seq., 562 et scq. 



