BRITISH ASSOCIATION. 269 



iy. — Form and relations of the last abdominal ring and its appendages. 



Although the characters of these latter have been long employed in 

 diagnosis, yet it would appear that their true homological value has 

 neither been appreciated nor understood : for, although in all the genera 

 of this group their presence has been recognised under the names of 

 " abdominal false feet," "caudal appendages" and "appendages of the 

 last segment" yet, as far as I am aware, the true relations subsisting be- 

 tween them and the so-called " thoracic" feet have been either over- 

 looked or only hinted at 



Their structure in all the genera is pretty much the same : a broad 

 basal joint, articulated somewhere at the termination of the last abdo- 

 minal ring, and furnished (except, perhaps, in Tylos, which I have 

 never seen) with a pair of dissimilar appendages, the external generally 

 broad, the internal pointed and linear, and inserted above the external ; 

 or, to speak more correctly, an appendage of two or more articulated 

 joints, the basal joint furnished at its inner side with an accessory ap- 

 pendage; that is, a foot, in which the second (the first being the poste- 

 rior angle of the ring), third, and fourth, &c, articulations are present, 

 the second, or ischium, being furnished with an appendage. 



That this is the true nature of the organ appears from the follow- 

 ing :— 



1st. The mode and point of articulation of the peduncle (basis) with 

 the last abdominal segment. 



2nd. The mode of articulation of the so-called external appendage 

 (ischium) and peduncle (basis). 



3rd. The difference in form and relation between the internal (ac- 

 cessory filament) and external (ischium) appendages. 



1st. The mode of articulation of the peduncle. 



By reference to the observations on the so-called " epimerals and 

 posterior angles of the abdominal rings," it will be seen that, taking into 

 consideration the ultimate segment only, two principal types exist, viz., 

 those in which " epimerals" or coxae exist, as Ligia, and those in which 

 these organs are absent, as Oniscus, Porcellio, and, perhaps, Philoscia, 

 &c 



Now, if we examine these, we will find that the point of articula- 

 tion between what we have called the peduncle, and the last abdominal 

 segment, is different in these two types. 



In the first (Ligia, for instance), it takes place at the posterior mar- 

 gin of the ring, in a notch formed between what I have considered as 

 the coxae and the posterior margin of the ring, by the whole superior 

 border of the peduncle, which is broad and flat, the exterior angle of 

 which forms a regular ball-and-socket joint, which is received into a 

 regular notch in the posterior angle of the ring, exactly similar to the 

 notch on the under side of the " epimeral" of the cephalo-thoracic 

 ring. 



In the other (Oniscus, to wit), the inferior portion of the peduncle is 

 narrowed, so that it appears to be articulated by the exterior angle only, 



VOL. iv. 2 u 



