MICROSCOPICAL STUDIES. 21 



remnant of the slit-perforated branchial pharynx, so well developed 

 in the simple Ascidians, such as Ascidia and Giona, the slits 

 having gradually coalesced, and being now represented by the two 

 great spaces which lie on either side of the branchial septum. 

 Recent research (Brooks' " Salpa in its relation to the Evolution of 

 Life," Baltimore, 1893), however, makes it probable that the an- 

 cestors of Salpa had never a complicated system of branchial clefts, 

 and that the present represents the little changed primitive plan. 

 At first, as Prof. Brooks points out, the ancestral Salp had an elon- 

 gated digestive tube without pharyngeal clefts. The anterior part was 

 distended and ciliated. Later on slime cells appeared, to retain food- 

 particles swept in by the free current of water passing through the 

 body. But this stream would also be liable to carry partially 

 digested food particles away — so the appearance of one or more 

 clefts in the distended pharyngeal part, allowing the water, after 

 bathing the slime covered parts, to pass freely away without coursing 

 through the intestine, would be a distinct advantage and would be 

 retained by natural selection'. The chamber anterior to the so-called 

 branchial septum, therefore represents the pharynx — while the pos- 

 terior chamber is the cloaca (cl), into which also empties the ali- 

 mentary canal by the anus. On the ventral surface of the pharynx 

 there stretches longitudinally a deep gutter with glandular walls, the 

 upper edges of which join along the greater portion of its length. 

 This appears optically to be a rod and is the endostyle (end) ; at 

 its front extremity it sends off two diverging ciliated bands (ac)^— 

 the peripharyngeal bands or ciliated arc — which, arching upwards, 

 join again and pass into the branchial septum. The cavity of the 

 endostyle is ciliated, and the slime secreted by its glandular walls, is 

 passed on by the cilia into the peripharyngeal bands, and thence to 

 the ciliated surface of the branchial septum. At the posterior 

 extremity of the pharynx is the short oesophagus, passing into a wide 

 stomach ; thence a short intestine leads to the cloaca (cl). This 

 digestive tract in all pelagic tunicates is concentrated into a very 

 small space, and forms the only opaque mass in the body, and is the 

 so-called nucleus (nc). Water passing in at the mouth laden with 

 food particles, has the latter arrested by the slime covered ciliated 

 arc and branchial septum. Passing through the two gill clefts the 

 water finds its way into the cloaca, whence it is discharged by the 

 exhalent aperture. The food particles arrested are sent by ciliary 

 action down the branchial septum and into the oesophagus. 



The animals move through the water by repeated violent expul- 

 sions of water, due to contraction of the muscular hoops. According 

 as the water is expelled from mouth or exhalent aperture, the 

 movement is backwards or forwards. 



