564 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



indicate a step higher in organization and a beginning of a specialization 

 of certain tissues for carrying on the functions of respiration. This is 

 the case in the polyps and in the sea-anemone, while in the bryozoa 

 there is a marked dilatation of the pharynx, which seems to be particu- 

 larly intended to provide for the aeration of fluids. 



In certain members of the group of annuloida a still higher special- 

 ization of the respiratory apparatus is met with. Thus, in many there 

 exist peculiar ramified contractile vessels, the trunks of which open 

 upon the surface of the body, and are in part ciliated in their interior. 

 These are the so-called water-vessels, and are supposed to be subservient 

 to the respiratory process. 



In various of the echinoderms, in addition to numerous gill-like 

 fringes, two sets of canals are found, the one carrying the nutritive fluid, 



and the other radiating from a ring 

 around the mouth, and distributing 

 aerated water. 



In worms, especially in the 

 silk-worm, a system of spiral vessels 

 analogous to those of the plant and 

 permeating the structure in every 

 direction is often to be detected. 

 These are called tracheae, and com- 

 municate directly with the atmos- 

 phere by open breathing orifices on 

 different portions of the body of the 



insect, and may be readily noticed 

 FIG. 238. TRACHEAE OF INSECT, SHOWING J 



THE SPIRAL FIBRE. (Jeffrey Bell.) i n the caterpillar as dark spots upon 



the sides. In fresh-water worms, 



like the leech and earth-worm, the body is covered externally by a viscid 

 fluid which has the power of absorbing air; so such animals breathe by 

 the skin, beneath which lies a dense net-work of blood-vessels. In 

 insects one of these spiracles, as they are also named, traverses the 

 body on either side along its whole length, sending out ramifications, as 

 referred to above. The tracheae are prevented from having their cavities 

 obliterated by spiral elastic fibres which seem an analogue of the carti- 

 laginous rings in the tracheae and bronchise of the air-breathing verte- 

 brates (Fig. 238). 



Thus, in insects, as in mammals, the air is carried to the fluids to be 

 aerated. 



In marine worms, which are water-breathing animals, the simplest 

 form of gill is seen ; it consists of delicate veins projecting through 

 the skin along the side of the body in a series of arborescent tufts. As 

 these float in the water the blood is purified (Figs. 239 and 240). 



