14 PHYLUM TUNICATA (UROCHORDA). 



its hind end or anteriorly. In the latter case the rectum is of 

 considerable extent and lies along the dorsal wall of the pharynx 

 adjacent to the dorsal lamina (Figs. 3 and 12). 



The oesophagus leaves the pharynx dorsally, usually postero- 

 dorsally, in the middle line at or near the end of the dorsal 

 lamina. It passes into the dilated stomach, the other end of 

 which is continued as the intestine which finally passes into the 

 rectum. There is very generally present a gland which consists 

 of fine colourless tubules ramifying over the stomach and intes- 

 tine and opening into the pyloric end of the stomach or first part 

 of the intestine ; it is called the pyloric gland (hyaline organ). 

 In a few forms (Molgulidae, many Cynthiidae, etc.) glandular 

 masses which have been compared to a liver, are found on the 

 walls of the stomach. A longitudinal fold (typhlosole) of the 

 intestinal wall, projecting into the lumen of the intestine, is often 

 present. The epithelium of the digestive tube is partly ciliated 

 and partly glandular. 



The central nervous system consists of an elongated ganglion 

 embedded in the mantle on the dorsal surface of the body be- 

 tween the mouth and atrial aperture (Fig. 3, 14, Fig. 5, g). It 

 gives off nerves from its front and hind ends. A median pos- 

 terior nerve containing nerve cells is present in many forms : it 

 is called the visceral nerve and is supposed to be the posterior 

 part of the nerve cord of the larva. The ganglion is solid and 

 consists of nerve fibres in the centre and nerve cells at the 

 periphery. 



Sense organs. Organs of general sensation in the form of 

 tactile hairs are always present. But pxcept in Thaliacea and 

 Appendiculariae, and possibly Pyroscma, visual and auditory 

 organs are not found. The red pigment spots placed between the 

 lobes of the mouth and of the atrial aperture may however 

 have some visual function. 



The salps and Pyrosoma possess phosphorescent organs. 



The coelom of the Tunicata is not thoroughly understood. 

 The general spaces and sinuses of the body are not coelomic but 

 vascular (haemocoelic). They are frequently spoken of as con- 

 stituting the primary body-cavity and as being a persistent part 

 of the segmentation cavity of the early embryo. In our opinion 

 however this nomenclature is misleading and as a matter of 

 fact the reputed embryonic derivation on which the terminology 



