412 UROCHORDA [CH. XVIII 



the centre called the cloaca. Pyrosoma, is a free-floating colonial 

 form, with the shape of a cylinder open at both ends, the atrial 

 cavities of the constituent persons opening on the inner surface, 

 their mouths on the outer. Its name (lit. fire-body) is derived from 

 the circumstance that it is brilliantly phosphorescent. 



The Thaliacea are extraordinary forms. They have the shape 

 of cylinders with the mouth at one end and the atrial opening 

 at the other, and their body is surrounded wholly or partly 

 with muscular hoops like the hoops encasing a barrel. The 

 commonest form is Salpa, which at intervals may be seen in 

 countless numbers swimming at the surface of the sea. In this 



/ 8 

 9 .16 



FIG. 206, Semi-diagrammatic view of left side of Salpa. From Herdman. 

 1. Branchial aperture. 2. Atrial aperture. 3. Anus. 4. Branchial sac. 

 5. "Gill." 6. Subneural gland. 7. Endostyle. 8. Heart. 



9. Oesophagus. 11. Languet. 12. Muscle bands. 13. Nerve 

 ganglion. 14. Embryo in ovisac. 15. Peribranchial cavity. 



16. Peripharyngeal band. 17. Stomach. 18. Testes. 19. Test. 

 20. Subneural gland. 



animal the test is of a glassy transparency. The two original 

 atrial openings or gill-slits of the larva do not become divided by 

 partitions, but develop into two huge vacuities in the side walls 

 of the pharynx, reducing its dorsal wall to a mere band, the 

 so-called "gill." There are two distinct forms of this animal, a sexual 

 and an asexual, one giving rise to the other, so that here we have 

 a case of "alternation of generations." In the asexual form we 

 find an endostyle-process or stolon which gives rise to a chain of 

 small sexual forms which one by one drop off. Each sexual form 

 produces only one egg. This when fertilised does not give rise to 

 a tailed larva, but becomes attached to the atrial wall of the 

 mother by a knob of maternal tissue containing blood-vessels, called 

 the placenta, which is embedded in a disc of embryonic tissue, 

 through which nourishment diffuses from mother to embryo. In 

 this position the embryo grows up into an asexual form and 

 eventually breaks loose and swims away. 



