XVIII] 



RESPIRATORY AND CIRCULATORY SYSTEMS 



389 



connection with the ectoderm through several strings of cells with 

 a tibrillar sheath, known as dorsal roots (18, Fig. 183). 



There are very numerous gill-slits opening into the alimentary 

 canal, in the front part of the trunk region ; they ought rather to 

 be called pouches with a small outer and a large inner opening. 

 The inner opening of each pouch is divided almost into two by a 

 tongue projecting down from its dorsal edge, the so-called tongue- 

 bar. This tongue-bar is specially richly supplied with blood- 

 vessels, especially on its outer surface, and so may be regarded as 

 the principal respiratory organ. It may be doubted indeed if the 

 principal function of the gill-clefts in the Balanoglossida be really 

 respiratory. These openings seemrather 

 to be intended to filter off the water 

 from the wet mud which is swallowed 

 as food. The blood-vessels are destitute 

 in most cases of any proper wall : they 

 are as it were mere crevices between the 

 epithelial walls of the gut, coelom and 

 skin. There is however a well-defined 

 contractile dorsal channel running for- 

 ward into the kidney, the contractility 

 being confined to the front end in the 

 proboscis, where there is a closed sac 

 with muscular walls, which pulsate 

 rhythmically, situated above the blood- 

 vessel. The sac is termed the peri- 

 cardium, and the dilated part of the 

 blood-vessel below it, the heart. This 

 dorsal vessel communicates with a ven- 

 tral vessel in the trunk region by two 

 descending curved vessels at the sides 

 of the collar. Each of the coelomie 

 cavities of the trunk sends forwards 

 into the collar region a narrow tongue 

 lying at the side of the blood-vessel. 

 These tubes from their relation to the 

 vessel are called perihaemal tubes 

 (Gr. TrepL, around ; at/xa, blood). 



The genital organs or gonads are 

 mere packets of cells in the gill region 

 and behind it, developed from the wall of the trunk coelom (Fig. 184). 

 Each when ripe forms its own opening through the body-wall. 



14 



,> 15 



FIG. 184. Longitudinal hori- 

 zontal section through Glos- 

 sobalanus. Diagrammatic. 



1. Proboscis. 2. Collar. 3. Trunk. 

 4. Proboscis cavity. 5. Glo- 

 merulus. 6. Pericardium. 

 7. Heart. 8. Proboscis pore. 

 9. Collar cavity. 10. Peri- 

 haemal cavity. 11. Collar 

 pore. 12. Dorsal blood- 

 vessel. 13. Alimentary canal. 

 14. Branchial sac with ex- 

 ternal opening. 15. Repro- 

 ductive organs. 



