12 MORPHOLOGICAL TYPES 



nearly as far as the anus. The atrium has been formed by 

 folds of the body- wall above the gill-slits growing down on 

 each side (the metapleural folds) and meeting underneath 

 what is the true ventral surface of the animal. The space of 

 the atrium therefore represents a portion of the outside world, 

 and is lined entirely by ectoderm. The low ridges running 

 along the ventro-lateral edge of each of the atrial folds are the 

 metapleural folds, and between them, meeting in the middle 

 line, are the epipleurs, in the form of horizontal shelves. These 

 close off the atrial cavity (see p. 170). The atrium is closed 

 in front so that all the water which enters it does so through 

 the gill-slits and passes out of the atriopore. Where the 

 pharynx passes into the intestine, a pair of conical outpushings 

 of the atrium project into the dorsal ccelomic cavities, one on 

 each side, forming the so-called " brown funnels " (see p. 15). 

 The function of the atrium is to protect the pharyngeal 

 region, which is very vulnerable owing to the gill-slits. 



Respiratory System. — At early stages the gill-slits corre- 

 sponded to the segmentation of the body, but more and more 

 of them are formed (up to 180) and the correspondence is 

 lost. The gill-slits are separated from each other by gill-bars, 

 the inner surface of which is covered by endodermal, the 

 outer by ectodermal tissue (forming the inner wall of the 

 atrium). There are two kinds of gill-bars : primary, and 

 secondary or tongue-bars. All the bars have a skeletal rod 

 (composed of a chitin-like substance) passing down them and 

 stiffening them. The rods of the secondary bars end simply 

 at their ventral ends, while those of the primary bars bifurcate. 

 Another difference is that the primary bars contain a portion 

 of ccelomic cavity while the secondary bars do not. The bars 

 are strongly ciliated, and by the activity of these cilia water is 

 forced through the slits into the atrium. As the water passes 

 between the gill-bars, the blood circulating in the blood-vessels 

 of the latter becomes oxygenated. There are three vessels in 

 each primary bar and two in each secondary bar ; the vessels 

 in the secondary bars are connected with those in the primary 

 bars by vessels running in the synapticula, or connecting 

 struts. 



