TYPE PBOTOORORDATA. 



609 



front of the anterior end of tlie paired ventral fin is the 

 atrial pore (p), which leads into a cavity termed the atrium 

 (6). The outer walls of this cavity arise as folds of the 

 sides of the body, termed the epipleural folds, which, in- 

 creasing in size, fuse together below except at the atrial 

 pore, thus enclosing the atrial cavity (Fig. 279, b), which is 

 lined throughout by ectoderm and surrounds the sides and 

 ventral surface of the anterior two thirds of the body. Ante- 

 riorly the cavity is closed by the fusion of the folds with the 

 body-wall in the neighborhood of the larval mouth, but in 

 front of this is a hood-shaped fold which arises independ- 

 ently of the atrial folds and forms the oral hood, at the bot- 



Pig. 278. — AmpMoxus lanceolatus (after Boteri from Hertwig). 



a = anus. m — muscles. 



ail = eye. n = nephridium. 



b = atrial chamber. o — mouth. 



ch = notochord. p = atrial pore. 



g = reproductive organ. r = dorsal nerve-cord. 



I = liver. sp = branchial cleft. 



torn of which lies the original mouth, the margins of the hood 

 surrounding what may be termed for distinction the adult 

 mouth (Fig. 278, o). These margins are produced into a 

 number of tentacular processes, termed cirrhi, each one of 

 which contains an axial supportive rod borne by a skeletal 

 ring surrounding the adult mouth, and each has its surface 

 raised into numerous sensory papillae. 



The ectoderm is very simple in its character, consisting of 

 a single layer of cells resting below upon a well-developed 

 layer of connective tissue. The arrangement of the coelomic 

 cavities is by no means simple, however, and may be best 

 understood by considering their mode of development. In a 

 very young embryo a fold may be seen extending along each 

 side of the primitive digestive tract on its dorsal surface. In 



