138 



ESSENTIALS OF ZOOLOGY 



Eespiration is brought about by the gills removing oxygen 

 from the water passing over them, and by the blood taking 

 the oxygen up in exchange for such gaseous waste matter 

 as can be dealt with by the gills. There is evidently no re- 

 spiratory pigment, and the oxygen must therefore be simply 

 dissolved in the plasma. 



The atrial cavity ends blindly in front and opens posteriorly 

 at the atriopore, beyond which it extends a short distance on 

 the right side of the intestine. The water which is filtered 



through the gill slits is passed 

 into the cavity and is dis- 

 charged at the atriopore. In 

 the young the gills open 

 directly to the exterior, and 

 the atrial cavity is developed 

 by a median ventral infolding 

 of the body wall. 



A coelom is developed as 

 a series of paired vesicles from 

 the endoderm, and these are 

 resolved into the somites, each 

 of which contains a cavity 

 called the myocoel, and into 

 the body cavity, or splanch- 

 nocoel. The inner or visceral 



layer of the splanchnocoel enwraps the alimentary canal, and 

 the parietal layer lines the body wall. This arrangement is 

 preserved in the posterior region of the body, but in the 

 region of the wall of the atrial cavity and the pharynx the 

 cavity is divided into a series of tubes and spaces. Of these 

 the coelomic spaces on each side of the dorsal region of the 

 pharynx may be observed in sections, and the small canals of 

 the coelom which traverse the gills from the endostyle to the 

 dorsal coelomic spaces just mentioned. An independent 

 space, the preoral cavity, occupies and distends the proboscis 

 below the notochord. It is derived from the right head 

 cavity of the larva, and the left one is reduced to form the 

 inner part of Hatschek's pit, which opens on the roof of the 

 mouth (see p. 149). 



Dermal layer 



Muscle layer 

 Nerve cord 



}Sclerotome 

 Notochord 



Aorta 



Intestine 

 Somatopleure 

 Splanchnopleure 

 Subintestinal 

 vein 



FIG. 64. Amphioxus. Diagram- 

 matic transverse section of 

 abdominal region of larva. 



