VERTEBRATA ACRANIA. 139 1 



attached below to a ridge running along the top of the neural 

 sheath, and projecting above into box-like spaces full of lymph. 

 The anal fin is supported by a double series of such fin-rays. 



4. Digestive and Respiratory Organs (Figs. 38 and 39). The 

 gut is a straight tube running from mouth to anus, and consisting 

 of buccal cavity, respiratory pharynx, gullet, stomach with liver, 

 and intestine. It is ciliated throughout. 



The buccal cavity, into which the wide jawless mouth opens, is 

 somewhat funnel-shaped, and it is lined by two kinds of epithelium,, 

 the boundary between which is marked by a series of lobes. The 

 kind which occurs in the posterior part of the cavity is distin- 

 guished by the presence of pigment and specially long cilia. At 

 the back of the buccal cavity there is a muscular partition, the 

 velum, which is perforated by an aperture leading into the pharynx 

 and guarded by a circlet of twelve delicate backwardly projecting 

 tentacles. Between buccal cavity and atriopore the gut is sus- 

 pended from the sheath of the notochord in a spacious atrial 

 cavity. 



The pharynx is the largest and most characteristic part of the 

 gut, extending for about half its length as a wide tube laterally 

 perforated by numerous oblique gill-slits, opening into the sur- 

 rounding atrial cavity. The first formed gill-slits in the young 

 larva open at first directly to the exterior, and then into a longi- 

 tudinal groove, the sides of which unite to form a tube open 

 behind. This tube, the opening of which persists as the atriopore, 

 gradually becomes more extensive, sinking into the body, so to 

 speak, and surrounding the gut (except on the dorsal side) behind 

 the buccal cavity. There is also a narrow prolongation of the 

 atrial cavity extending back on the right side of the intestine 

 between atriopore and anus. It is obvious from the above outline 

 of its development, that the atrial cavity is, morphologically, a 

 part of the exterior. 



The cavity of the pharynx is much wider in its anterior than 

 in its posterior portion, where it is somewhat flattened from side 

 to side. A well-marked Abranchial groove runs along the middle 

 of the roof of the pharynx, while in the median line of the floor 

 there is a thickening known as the endostyle, flat or convex ante- 

 riorly, groove-like posteriorly. Both these median regions are 

 characterized by the presence of elongated columnar epithelial 

 cells provided with very long cilia. 



