Respiration in Invertebrate Animals. 413 



nature. The cell or the fibre, which the wondrous microscope 

 only can reveal to human ken, is no less fixed and invariable in 

 its structure than the huge bone or the stupendous brain. 



In the Patella the heart stands in the geometrical centre of 

 the body. It is situated above and behind the head. It is not 

 perforated by the intestiacs as it is in the Bivalves. It is an 

 elongated sac dividing in front into two main pallial trunks. 

 These latter distribute arterial blood throughout the mantle*. 

 From the mantle and the viscera it returns into the branchiae 

 and thence into the heart, to be redistributed over the body. 

 This apparatus can be detected with perfect clearness in the 

 uninfected subject. 



In Patella the branchise (fig. 5 a, a) form a circle, which 

 is interrupted only by a small notch for the admission of 

 water. That is, the lamellaj are neither deficient behind nor 

 before. The " cordon '^ is continued over that portion of the 

 margin of the mantle which is situated in advance of the head. 

 Thus, in Patella the branchiae neither arise from, nor are in any 

 way attached to, the neck of the animal. They are developments 

 of the mantle alone. This point is one of specific importance. 

 It proves that the figure used by the late Prof. E. Forbes was 

 unphilosophical, because unsupported by anatomy. He said that 

 the branchiae of Patella were really only those of Fissurella and 

 Haliotis, fixed to the mantle and extended all round, instead of 

 being free plumes as in the latter. But it is at once obvious, 

 that not only the branchiae themselves, but the anatomical rela- 

 tions of them are radically diff'erent in Patella and Fissurella and 

 Haliotis for example. If, indeed, the latter genera have no better 

 title to a rank in the Patellidan group than that which is fur- 

 nished by the branchiae, they should receive at once a summary 

 sentence of exclusion. 



The branchial organ of Patella consists of a double row of 

 leaves (fig. 5 a, e) oblong in shape (fig. 8), standing vertically 

 on, and at right angles with the plane of, the mantle (fig. 5 c, c). 

 They constitute a special apparatus distinct from that papillose, 

 ciliated fringe {d) with which the extreme edge of the mantle is 

 ornamented. They extend over the entire circumference of the 

 pallial border. They are not attached to any part of the body 

 of the animal. The outermost row of leaflets {a) is a little larger 



* So clear and water-like are the fluid contents of these vessels in a 

 fresh specimen, while expanding itself in the struggle to get out of its shell, 

 that they may most readily, but most erroneously, be mistaken for aqui- 

 ferous canals. As on a future occasion I shall have a great deal to say on 

 the ill-understood subject of the aquiferous system of Mollusks, at present 

 I only desire to indicate a source of misconception which has led many an 

 acute naturalist into error. 



