NOTE ON THE CILIATED PIT OF ASCIDIANS. 143 



dorsal line of the pharynx a long way into it ; a rod of skeletal 

 tissue (fig. 5, s/c. b.) is present near its base, and extends 

 throughout its whole length. 



The gill-slits are very numerous. The bars between them 

 are composed of connective tissue, which is a part of the 

 pseudoecele, and contains a great number of blood-corpuscles 

 (figs. 5 and 7, br. b.). 



The skeletal system is somewhat complicated. The slits 

 (fig. 7, br. s.) are elongated longitudinally, and arranged in 

 transverse rows. The rows are separated from one another by 

 thick skeletal rods, and divided into sets of five by similar rods 

 (fig. 7, sk. b.), which meet the former at right angles. In 

 addition to this main skeletal system, there is a system of finer 

 rods (fig. 7, s. sk. b.) which accompany the larger ones, and are 

 connected together by longitudinal rods passing between every 

 two gill-slits, and a transverse rod lying across the centre of 

 each row of slits. This secondary system of rods lies on the 

 internal face of the pharynx. 



The bars themselves in transverse section (fig. 5, br. b.) are 

 seen to be covered on the surfaces turned towards the slits by 

 columnar cells provided with cilia, and on their inner and 

 outer walls by fiat non-ciliated cells. Their internal cavity, 

 as already stated, contains a blood sinus. 



The (Esophagus (fig. 4, (Es.) is very short and simple, 

 being lined with short, columnar, ciliated cells. 



The Stomach. — The walls of the stomach are thrown up 

 into deep glands, which run in a longitudinal direction along 

 its whole length. 



The mouths of the glands are lined by high columnar 

 ciliated cells (fig. 6, c. c. s.), very closely packed together, with 

 a smallish nucleus placed at about the centre of each cell. 

 The portion of the cells towards the lumen stains very deeply. 



These cells are separated from those forming the deeper part 

 of the gland by a slight constriction (fig. 6, c). 



The latter (fig. 6, s. c. s.) are somewhat higher than the 

 former, and are not ciliated. The nuclei are placed quite near 

 the bases of the cells. They appear to be secretory cells, as 



