THE LOBSTER AS A TYPE OF CRUSTACEA 19 



to the side-wall of the thorax itself. Each gill is 

 somewhat like a bottle-brush in shape, consisting 

 of a central stalk set round with rows of soft hair- 

 like processes. As the blood streams through the 

 minute channels inside these filaments, it is separated 

 only by a thin membrane from the surrounding 

 water, and the absorption of oxygen and discharge 

 of carbon dioxide can go on easily. For this purpose, 

 however, it is necessary that the water within the 

 gill chamber should be constantly renewed, and this 

 is effected in the following way : the front part of 

 the gill chamber forms a narrow channel running 

 forward under the side - wall of the carapace. 

 Within this channel lies a large plate known as the 

 scaphognathite, attached to the outer side of the 

 maxilla, which during life is constantly in move- 

 ment, causing a current of water to flow forwards 

 through the channel. The water enters the gill 

 chamber by the narrow slit-like space between the 

 lower edge of the carapace and the bases of the 

 legs, and is discharged in front at the sides of the 

 head, where its movement is helped by the vibrating 

 exopodites of the maxillipeds. 



At the sides of the stomach, in the front part of 

 the head, lie a pair of glands which, from their 

 colour, are known as the green glands. These are 

 the excretory organs, corresponding in function to 

 the kidneys of the higher animals. Each has con- 

 nected with it a thin-walled bladder, which opens to 



