28 Charles Russell Bardeen 



Organs of Special Sense — The nasal fossae are patent but 

 irregular in form (sections h and c). No olfactory nerve can be 

 distinguished. The eyes are very rudimentary (section d). The 

 lens consists of a small round clump of cells. The optic stalk is 

 patent and the pigment and sensory layers of the retina are separ- 

 ated by a space connected with the lumen of the eye stalk. The 

 sensory layer consists of a thin membrane of partly degenerate cells. 

 The cells of the pigment layer are irregular in outline. The audi- 

 tory vesicles are small round sacs from each of which a short dorsal 

 diverticulum extends. The auditory nerve is not distinguishable. 



Peripheral Nerves — The ganglion of the trigeminal nerve is 

 partly developed. Those of the ninth and tenth nerves are not 

 distinct. No spinal ganglia can be distinguished with the excep- 

 tion of one small group of cells in the mid-thoracic region. 



Alimentary Canal — An irregularly shaped mouth extends from 

 the dorsal surface of the head to the pharynx. Its margis are 

 surrounded by folds of tissue (section a). The pharynx is dilated 

 near the mouth (section h). Posteriorly it is flattened from front 

 to back (sections c and d). The gill clefts are patent. From the 

 septa which partially close them irregular outgrowths arise (sec- 

 tion d). The oesophagus passes posteriorly from the right side of 

 the pharynx (section e). At the left of the origin of the oesophagus 

 there is a mass of cells, flat in cross section, which is continued for 

 some distance along the dorsal wall of the body cavity (section 

 /). This represents the tracheo-pulmonary anlage. The oesoph- 

 agus passes into the stomach at the left of the liver (section e). 

 The gut curves in front of the pancreas (section /), whence it is con- 

 tinued into a straight gut which about the middle of the trunk ex- 

 hibits a distinct lumen that is continued with some interruptions to 

 the rectum (sections h and i ) and through the anus to the exterior. 

 In addition to the main lumer^ several irregular tubular spaces 

 occur in the mass of cells which compose the gut. The greatly 

 dilated body cavity extends far forward beneath the pharynx. 

 The liver is partially diff'erentiated, extends forward beneath the 

 pharynx and posterior to the rudiment of the heart has a few blood 

 sinuses containing blood corpuscles. The pancreas is partially 

 differentiated posterior to the liver and dorsal to the stomach. In 



