516 VEKRUCID.E. 



the shell together with the circumference of the basal 

 membrane must be subjected to movement, but the shell 

 is united to the basal membrane by corium and by trans- 

 parent structureless chitine (both of which may be left 

 out of question) and by a circle of short fibres, which 

 adhere at their lower ends to the firmlv cemented circular 

 zone, and by their upper ends to the shell ; and these fibres 

 have been very carefully examined by Professor Quekett, 

 and pronounced to be not muscular, but exclusively liga- 

 mentous, and therefore incapable of moving the edge of 

 the shell. The basal membrane over the central hollow is, 

 as stated, quite loose ; its lower surface, formed by a re- 

 ticulated layer of horny cement-tissue, shows no signs of 

 abrasion, and the membrane is so brittle and tender, that 

 in specimens which have been once dried and then well 

 soaked, it almost invariably cracks when the shell is re- 

 moved, owing to its mere adhesion to the delicate inner 

 tunics of the sack; yet on the mechanical theory, the 

 wearing of the central hollow must have been caused by the 

 action of this middle portion of the basal membrane,* 

 which, it may be repeated, is destitute of muscles. From 

 the presence of the prehensile pupal antennae, enveloped 

 in cement) nearly in the centre of the basal membrane, 

 it is certain that this spot w T as originally attached to the 

 supporting surface, and has since been detached from it; 

 as, moreover, the central hollow goes on increasing in 

 diameter with the growth of the shell, it is certain that the 

 inner edge of the firmly attached circular zone of basal 

 membrane must likewise continually go on becoming de- 

 tached : it may, then, be asked by what force can the basal 

 membrane, seeing that it is united to its own shell above 

 only by fibres of ligament near the circumference, be con- 

 tinually torn away from the underlying support, to which 



* Mr. Hancock suggests to me that the basal membrane, on the mechanical 

 theory, need not itself move; the motion of epithelial scales, were they trans- 

 ferred into cutting agents, might be supposed to be sufficient. But of sucli 

 scales, though I used very high powers, I could see no trace ; and their pre- 

 sence on the under side of the layer of cement seems hardly possible. More- 

 over, according to Von Siebold ('Anatomic Comparee,' torn. 1, p. 412), ciliary 

 action has not been observed in any Crustacean, or indeed any Articulate animal. 

 This same statement is likewise made in Annals and Mag. of Nat. Hist.lSoi, 

 p. 136, by Dr. T. Williams. 



