Res^nration in Invertebrate Animals. 419 



and less distinguishable. In other words^ the layers are so inti- 

 mately soldered together as almost to obliterate the intervening 

 space. The layers nevertheless carry separate and distinct 

 planes of blood-channels. In Fissurella the organs of breathing 

 (fig. 10 a, a) consist of two plumose tapering processes {a, a), 

 projecting forwards (when fully expanded) to a considerable 

 distance under the anterior vault of the shell. They are capable, 

 like the arms of a Brachiopod, of being coiled up on themselves. 

 They are distinguished from those of Emarginula in having a 

 double system of leaves. These gills therefore present rather a 

 pinnate than a pectinate figure. They stand at the sides of the 

 chamber, the roof of which is perforated by the opening at the 

 apex of the shell. This orifice admits fresh water into this space ; 

 it also conveys externally all refuse excretions. In the dispo- 

 sition of the vessels, in the character of the ciliary epithelium, 

 in the fact of two planes of vessels, the branchiae of Fissurella 

 conform to the standard of the Patellan giU. 



The branchial plume of Acnieea testudinalis extends to a great 

 distance beyond the limits of the shell. It is fixed to the neck 

 of the animal at the root of the right tentacle. It is bisymme- 

 trically foliated ; each lamina in its xiltimate anatomy follows the 

 Patelloid type. The gills of Propilidium ancyloide consist of 

 two small feathery processes attached to the dorsal surface of 

 the neck of the animal. They incline in a parallel direction 

 to the right side of the cloacal cavity. They are free processes, 

 and furnished with a double series of leaves, supported by an 

 axial base, in which the afi"erent and efi'erent branchial vessels 

 are lodged. 



In Puncturella noachina the gill is non-symmetrical. It occu- 

 pies a special and spacious chamber on the left side of the neck 

 and foot. It is a pinnate structure highly ciliated, the lamina 

 being large. 



Judged by the structure and relations of the branchial system 

 exclusively, the Fissurellidse are undoubtedly placed in their true 

 serial position by Forbes and Hanley, namely intermedially be- 

 tween the Limpets and Haliotidae. They are Scutibranchiate. 

 This cannot be said of Acmaa and Propilidium. The branchia 

 here is not shielded by a fold of the mantle. Nor are they 

 Cyclobranchiate, for the branchial processes are free plumes. If 

 affinity is to be decided by structure rather than position, both 

 Acmcea and Propilidium should be classed with the Scutibranchs. 



The branchise in Haliotis (fig. 11) are sufficiently large to 

 enable us to trace satisfactorily the distribution of the larger 

 blood-channels. They will suffice to illustrate the circulatory 

 apparatus of the gills in all the other Scutibranchs. The 

 author^s account is drawn from an examination of a specimen 



28* 



