Prof. Allman on Chelura terebrans. 367 



ing above the first, must bring the appendages of the second 

 ring nearly vertically over those of the first ; and the episternal 

 pieces of the first ring remaining rudimental, the whole assumes 

 the appearance of a single plate supporting two appendages, one 

 on its upper, and the other on its lower edge. The third ring 

 becomes greatly developed, and carries its very large appendages 

 at its posterior end. 



The six pairs of abdominal appendages, though so very hetero- 

 morphous, may yet be easily reduced to a common type. Let us 

 assume as a point of comparison the natatory isomorphous legs 

 of the first three abdominal segments, in which we have the stem 

 and palp of a completely developed limb represented by a basal 

 inarticulate lamina supporting on its distal edge two long setose 

 lamellar rami with transverse strise indicating the traces of arti- 

 culation. In the fourth pair of abdominal members the basal 

 lamina becomes long and cylindrical, and the rami are reduced 

 to two minute inarticulate appendages ; in the fifth pair the basal 

 portion becomes again lamellar, disproportionately developed, and 

 produced on one side into a large fringed lobe, while the two ter- 

 minal rami assume the appearance of small oval fringed plates, 

 pushed towards the other side by the large lateral lobe ; finally, in 

 the sixth pair the basal portion preserves somewhat of its normal 

 condition while one of the rami disappears, and the other acquires 

 an enormous development, constituting one of the most striking 

 features in the little animaFs physiognomy ; in the seventh ab- 

 dominal segment the appendages have completely vanished. 



To the internal anatomy of Chelura I have not yet given suf- 

 ficient attention to enable me to introduce into the present paper 

 facts of much importance in this part of the inquiry. It may 

 however be stated, that a large dorsal vessel with transverse 

 branches may be seen vigorously pulsating through the entire 

 length of the thorax, and that the stomach is supported by a 

 corneous framework and lined with a minutely spinulose epi- 

 thelium. 



Chelura terebrans is an active little animal, swimming on its 

 back and employing its thoracic legs to adhere to the timber 

 which it has selected for its ravages. The large lamellar append- 

 ages placed near the anterior end of the great abdominal trunk 

 do not appear to be employed in swimming, they are kept thrown 

 upwards on each side of the spinous process of the third abdo- 

 minal segment, and seem in no way subservient to locomotion ; 

 they are not confined to any particular sex, and it is difiicult to 

 assign to them any office unless it be that of keeping the exca- 

 vations formed by the Amphipod free from the detritus of the 

 timber, and from other extraneous bodies which might interfere 

 with respiration. 



