414 ANIMAL RESPIRATION 



Brittle- Stars (Ophiuroids) are enclosed in very complete 

 scale-armour, so that special provision for breathing is a necessity. 

 There is a conspicuous swelling placed between the bases of 

 every two adjacent arms, and each of these contains a couple of 

 pouches opening by slits to the exterior. The ten pouches have 

 a ciliated lining, by means of which sea-water is made to flow 

 continuously through them for breathing purposes. And in some 

 cases the slit-like opening is divided into two holes, one of which 

 admits sea-water to the pouch, while the other serves as a means 

 of exit. 



Sea-Cucumbers (Holothurians) are leathery elongated forms 

 in which the external skeleton is reduced to detached plates 

 imbedded in the skin, so that a large part of the general surface 

 is available for breathing. In many of these creatures there is 

 a curious internal arrangement by which a great deal of the work 

 of respiration is effected. There are here, in many cases, two 

 large branched respiratory trees which open into the intestine 

 (fig. 545), through the external opening of which water is alter- 

 nately taken in and squeezed out, so that the large surface pre- 

 sented by the lining of the trees is constantly bathed with fresh 

 water, enabling exchange of carbonic acid gas for oxygen to be 

 readily carried on. 



THE RELATION OF THE WATER-VASCULAR SYSTEM 



TO BREATHING 



It is extremely probable that the water- vascular system as 

 we now find it was first evolved as a means of breathing, for in 

 all cases its radiating branches bear very numerous slender pro- 

 jections with thin walls, collectively presenting a very large 

 respiratory surface, making up for the area rendered useless for 

 this purpose by the development of firm calcareous plates in the 

 body-wall. 



Crinoids. — A typical case is presented by the Feather-Star 

 (Comatitld), in which five ciliated grooves run outwards from the 

 mouth to send branches along- the arms, and all their subdivisions. 

 As described elsewhere, the small particles and organisms which 

 constitute the food are conducted along these grooves to the 

 mouth (see p. 265). Examination of any part of one of the 

 arms shows that the food-groove which runs alono- it is flanked 



