12 PHYLUM TTJNICATA (UROCHORDA). 



ttidinal bars except the internal longitudinal or what are identified as the 

 internal longitudinal (Fig. 20). This condition has been interpreted as 

 being due to the absorption of the fine longitudinal bars and partial 

 confluence of the stigmata of a row. It may however be due to the simple 

 enlargement of the stigmata, no internal longitudinal bars being present. 

 The side walls of the pharynx are in some forms folded longitudinally 

 and the number of folds varies in the different types. 



The function of these various organs would appear to be as 

 follows. The glandular cells of the endostyle secrete a slimy 

 mucous substance which is moved forwards by ciliary currents 

 along the endostylar groove to the peripharyngeal band. Here 

 it is reinforced by mucus secreted by the gland cells present in 

 that organ, and kept in circular movement by ciliary action 

 round the entrance to the pharynx. While thus moving the 

 slime entangles within itself the minute organisms which enter 

 the mouth in the respiratory current of water, and from time to 

 time portions of it so charged become detached and pass down 

 along the dorsal lamina to the oesophageal opening. The main 

 body of water which enters the pharynx is thus deprived of its 

 floating contents and passed out through the gill-slits into the 

 atrial cavity and out by the atrial aperture. 



It is possible that a certain amount of slime from the hind end of the 

 endostyle passes back direct to the oesophagus along the retropharyngeal 

 band. It has also been suggested that the neural gland secretes mucus 

 which reinforces the endostylar mucus at the peripharyngeal band. 



The atrial cavity, or peribranchial cavity as it is sometimes 

 called, entirely surrounds that part of the pharynx which is 

 perforated by gill-slits except in the middle ventral line and for a 

 short distance at the anterior end of the dorsal lamina (Fig. 12). 

 It opens to the exterior by the atrial aperture and communicates 

 with the pharynx by the stigmata ; the anus and genital ducts 

 also open into it. Its lining epithelium which is ectodermal is 

 closely applied to the pharyngeal wall and is continuous through 

 the gill-slits with the endoderm of the pharynx. Water con- 

 tinually flows into it through the gill-slits and passes out by the 

 atrial aperture. It is traversed by vascular strands which pass 

 from the wall of the pharynx to the outer (mantle) wall, and it is 

 developed as two dorso- lateral in vagina t ions of ectoderm which 

 unite dorsally and extend laterally round the pharynx as far as 

 the endostyle. The dorsal part of it is frequently called the 

 cloaca. It is into this part that the anus and genital ducts open. 



