348 



INTRODUCTION TO ZOOLOGY 



CHAP. 



worm" can stay below for many hours, though at times it seems 



that it is necessary for it to 

 leave its mud- tube and swim 

 about near the surface in order 

 to take in oxygen from the 

 purer, better aerated surface 

 water, through the long gill 

 tubes filled with blood, which 

 project from the last segment 

 but one of the body, and prob- 

 ably also through the much 

 smaller but similar processes on 

 the last segment (Fig. 270). 

 The blood is red because of the 

 presence in it of haemoglobin 

 which doubtless acts here, as it 

 does in higher animals, as an 

 oxygen -carrier, very readily 

 combining with the oxygen in 

 the water and carrying it in the 

 blood to all parts of the body. 

 The circulation of the blood is 

 caused by the action of the 

 tubular heart which may be seen 

 pulsating near the hinder end of 

 the body on the dorsal side. 



It is to be noted that there is no 

 functional tracheal system in this 

 larva ; the air-tubes that are pre- 

 sent are rudimentary and closed. 

 FIG. 27Q.Chironomus Larva. The p a When the larval 



life is complete, the 



last larval skin is shed and the pupa appears, at first lying 

 half out of the mud-tube, and then, after a day or two, floating 

 up to the surface. It has a thickened head-end in which the 

 organs of the adult can be seen packed away below the pupal 

 skin (Fig. 271). There is a long segmented abdomen, the last 

 segment bearing a pair of processes, each fringed with long 

 stiff hairs which aid in the locomotion of the pupa. Re- 

 spiration takes place through two conspicuous bunches of 

 fine white hairs which project upwards from the front of 



