506 CHORDATA. 



acid, violet with chloriodide of zinc). Nowhere else among ani- 

 mals is there such a rich formation of cellulose. 



The anterior part of the digestive tract is modified into a 

 pharynx or branchial chamber, the walls of which become per- 

 forated with a varying number of gill slits, these leading either 

 directly to the exterior or, more usually, into a peribranchial 

 chamber, and from this to a cloaca or atrium (a), before reaching 

 the outside world. While the respiratory water passes through 

 the gill slits the food particles which it contains are received by a 

 ring-shaped ciliated band (peripharyngeal band) and, enveloped 

 by mucus, are ]ed to the oesophagus. This mucus is formed by a 

 ciliated glandular groove, the endostyle (e), on the ventral surface 

 of the pharynx. 



Between the gill region (end of the endostyle) and the stomach 

 lies the ventral heart enclosed in a pericardium. It has the 

 peculiarity, met nowhere else, of changing the direction of its 

 contractions at frequent intervals; after the heart has driven the 

 blood for a time to the gills it rests a while and then begins to 

 force the blood in the opposite direction, pumping it from the 

 gills and sending it towards the stomach. 



If we add to the foregoing that a dorsal ganglion and a her- 

 maphroditic gonad are present, the striking features of the group 

 are enumerated. The extreme forms, the Copelatae and the 

 Thaliacea, are rather remote, but they are connected by interme- 

 diate forms, the Ascidiae and Pyrosomas. 



Order I. Copelatae. 



These small forms one or a few centimeters in length are 

 pelagic; they have the anterior end inserted in a gelatinous envelope 

 or 'house' which replaces the lacking tunic. They swim like a 

 tadpole by means of a tail which arises from the hinder end of 

 the trunk. The alimentary canal (fig. 544) is bent on itself, and 

 both it and the two large gill slits, in contrast to all other tuni- 

 cates, open directly to the exterior. The heart (lacking only in 

 the Kowalewskidae) is ventral and the hermaphroditic gonads and 

 the nervous system dorsal. The latter consists of a cerebral 

 ganglion, with beside it an extremely simple auditory organ and a 

 ciliated groove, and farther a chain of ganglia extending into the 

 tail. The notochord, a gelatinous structure enclosed by a sheath 

 of cells, forms the skeletal axis of the tail ventral to the nerve cord 

 and gives attachment to muscles. Oikopleura,* Appendicularia,* 

 Fritillaria; Kowalewskia. 



