PHARYNX. 9 



which that at the posterior end is the largest. The marginal 

 ciliated bands are continuous in front with the peripharyngeal 

 band (posterior ridge, if both are present) and behind with 

 the ciliated ridge or groove (retropharyngeal band] which passes 

 from the hind end of the endostyle along the median line of the 

 posterior wall of the pharynx to the opening of the oesophagus. 



The retropharyngeal band may be a single ciliated ridge continued back 

 from one of the lips of the endostyle (most synascidians), or a single ridge 

 formed by the fusion of both margins (Molgulidae), or finally, as in dona, 

 both lips may be continued back with a groove between them. 



The dorsal lamina is, in most monascidians and many synas- 

 cidians, a fold of the dorsal pharyngeal wall containing blood 

 sinuses and extending from the anterior end of the pharynx to 

 the opening of the oesophagus. It may be continuous with or 

 separated from the peripharyngeal ridge (see above). The cells 

 covering it are somewhat more columnar than those over the 

 lateral walls of the pharynx and are for the most part ciliated. 

 In a few monascidians (e.g. dona) and some synascidians it has 

 the form of a series of isolated variously-shaped processes of the 

 dorsal wall, called languets (Fig. 3). These two forms of it are 

 connected by a condition in which it consists of a continuous 

 membrane carrying processes at intervals. The languets and 

 the processes last mentioned occur at the dorsal ends of the 

 transverse bars of the pharynx. The dorsal lamina, whether 

 consisting of languets or of a lamella, is curved, generally to the 

 right, so as to bound a groove leading to the oesophageal aper- 

 ture.. The posterior termination of the dorsal lamina varies 

 considerably and is often difficult to determine. It usually 

 seems to pass round the left side of the oesophageal opening and 

 become continuous with the retropharyngeal band, or it gradu- 

 ally dies away. 



The lateral walls of the pharynx have the form of a basket work, 

 and are pierced by numerous, usually longitudinally elongated 

 apertures, placed in transverse rows. These are the gill-slits or 

 stigmata. They are usually distributed all over the lateral 

 pharyngeal wall behind the peripharyngeal band, but are some- 

 times absent from its posterior part (Fig. 8). 



The number of transverse rows of stigmata and the number 

 of stigmata in a row are very variable, not only in different 

 species but also in individuals of the same species. It would 



