Respiration in Invertebrate Animals. 199 



of the water-current through the gill, above from the outside to 

 the inside, below from the inside to the outside. In a mecha- 

 nical point of view, there is much to admire in this contrivance. 

 The more forcibly the water is caused to pass through the 

 branchia, the more completely its delicate leaflets are straightened 

 and rendered smooth. All danger from mechanical injury is 

 thus obviated. 



The water is drawn into the hollow interior of the gill much 

 more slowly than it is driven out. No violence to the slender 

 structures of the organ can accrue from its forcible ejection, 

 since the spaces through which the water effects its exit are 

 bounded only by tendon and fibrous tissue. Now, it may be 

 asked with great reason, how is it that during the expiratory 

 shortening of the gill, the water does not again escape through 

 the passages by which it entered ? It is not difficult to answer this 

 question. When the large longitudinal muscle (fig. 1 h, h; fig. 2 c) 

 which is situated underneath the gill contracts, and thus ap- 

 proximates the two ends of the gill, it brings the dorsal half of 

 the organ closer together, forming the concavity of a curve, 

 while it separates the ventral half, which for the instant forms 

 the convexity of the curve. At this instant the expiratory cur- 

 rent escapes. Reversing this movement, it is not difficult to 

 perceive how the act of inspiration occurs. 



The gills of all Cephalopods are remarkably elastic; at one 

 moment exhibiting an extraordinary capability of dilatation, at 

 another of extreme contraction. This property is due to the 

 presence of muscular fibres. They are distributed throughout 

 the entire structure of the gill; they embrace the vessels; they 

 course along the edges of the laminae ; they are internally in- 

 termingled with elastic tissue, whose normal mode of action is 

 rhythmic ; they contribute in a most important manner to the 

 mechanism of breathing. 



The branchial system of the Cephalopods is distinguished by 

 one further peculiarity : — in no instance yet examined has the 

 presence of vibratile cilia been proved. In this particular they 

 are allied to the gills of the Crustacea and the Fish*. In the 

 Cephalopod, no part whatever of the gill is furnished with cilia. 

 At first it might be thought that its extreme flexibility super- 

 seded the necessity for such organules. The instance of the 

 Crustacean gill, however, which is perfectly passive, disproves 

 this imagination. IVhy cilia are denied to the branchiae of the 

 Cephalopod, cannot at present be explained. The fact is attested 

 by all observers. 



* I am sorry that, in consequence of the want of specimens, I cannot, from 

 personal knowledge, state at this moment whether the gills of the Pectini- 

 branchiata are ciliated or not. 



