Chap, vi.] GILLS OF CRUSTACEA. 225 



oxygen is, consequently, easily effected. The podo- 

 branchs and pleurobranchs are more elaborately con- 

 stituted than the simpler arthrobranchs. 



When we remember the well-known fact that the 

 Crustacea are altogether devoid of cilia, we find it at 

 first difficult to understand how water is driven 

 through the gill chamber ; we have only, however, to 

 make the experiment to see. that a current of water does 

 constantly enter at its hinder and pass out at the 

 anterior end. The apparatus by which this current is 

 produced is, again, a modification of one of the appen- 

 dages for the exopodite and epipodite of the second 

 maxilla (page 1 23) of either side is converted into a scoop- 

 shaped plate, the cavity of which is directed forwards, 

 and which itself fits into the anterior orifice of the 

 gill chamber ; this so-called scaphognatliite moves 

 backwards and forwards about 200 times a minute, 

 and with each backward and forward movement it 

 scoops out water at the anterior, and causes a fresh 

 supply of water to enter at the hinder end of the 

 animal ; moreover, the quicker the animal moves, 

 the quicker the action of the scaphognathite, and, in 

 consequence, the larger the inflow of oxygenated water. 

 Within the gill chamber the waving plumes of the 

 gills aid in the movement of the water, and the at- 

 tachment of the podobranchs to the ambulatory 

 (thoracic) and hinder mandibular appendages affords a 

 certainty that the more these appendages work the 

 greater will be the supply of oxygen that they will 

 receive. 



A few Crustacea are modified to breathe air. (See 

 below. ) 



Among the Arachnida, processes of the body 

 projecting into the water are found in the only mem- 

 bers of the group that are inhabitants of the sea; 

 these are the King-crabs, represented to-day by Li- 

 mulus. In these forms there is no protecting gill 

 p 16 



