Respiration in Inverteirute Animals. 409 



as that which subsists between those of Chiton and Fissurella. 

 The ultimate respiratory laminae in the last two are bilateral, 

 resting, that is, on either surface of a common axial plane. In 

 Emar<jinula and Patella they rise from one surface only of a 

 common basis. In Patella that basis is fixed, in Emarghnda it 

 is fiee. The branchial system of Chiton is distributed and sub- 

 divided, that of Fissurella is centralized in place and united in 

 structure. In anatomical arrangement they are precisely the 

 same. It should however be remembered, while discussing the 

 generic affinities of the Chitonidje, that, according to Cuvier^s * 

 dissections, the branchije in Chiton spinulosus (Linn.) are only 

 one-sidedly laminated, resembling the arrangement of the teeth 

 of a comb. By Forbes and Hanley f the branchiae in the British 

 families are defined as " forming a series of lamellce between the 

 mantle and the foot on each side." By Mr. Clark those of 

 Chiton fascicularis are described as consisting of " a cordon be- 

 tween the mantle and the foot, composed of fifteen oblique cord- 

 like, short, close-set, pale hvown filameuts on each side J." This 

 description is calculated to mislead. The branchiae in the Chi- 

 tons are neither * lamellae ' nor ' filaments.' They are complexly 

 constructed organs (PL XL figs. 2 & 3). Each 'filament' or 

 ' lamella ' (Forbes) is a separate and distinct fabric. Not less so 

 than one of the cones of the gill in the Crab, or one of the pen- 

 knife-shaped processes of the branchia of the Fish. In number 

 these conules differ according to the species. A row of fifteen 

 occurs in Chiton fasciculai'is , of seventeen in C. cinereus, of only 

 ten in C. asellus, on each side of the body. They amount to 

 eighteen in Chiton discrepans, to twelve only in C. ruber, to 

 eighteen in C. quinquevalvis (Brown), and even to twenty-four 

 on either side of the body in C. marmoreus. Several other spe- 

 cies of Chitons are described in works on British Mollusca. In 

 all, the branchiae are overlooked. Numerous comparisons of the 

 young with the old would be necessary to prove that in the 

 adult state of each species these organules are constant cha- 

 racters. If they are, in descriptions of species a correct account 

 of them, as regards number, size and position, should be in- 

 cluded. They are well known to vary in size, apparent form, and 

 in situation, relatively to the transverse median line of the body 

 in difi'erent species, but in plan of formation or design they pre- 

 sent no diversities. A branchia (figs. 2 & 3) in the genus Chiton 

 may be typically described as a process of the mantle, consisting 

 of a wedge-shaped axis (6), from whose opposite plane surfaces 



* Mem. sur les MoUusques ; aud Regne Animal, pi. (!)9'. vol. sur les Mol- 

 lusques. 



t British Mollusca, vol. ii. p. 3.91. 



X British Marine Testaceous Mollusca, p. 249. 



