r)() THE APODOUS HOLOTHURIANS 



and body-cavity, which are very short tubes, lined with a prominent ciliated 

 epithelium. Owing to this direct connection between the lumen of the water- 

 vascular system and the body-cavity, the fluids contained in the two are essen- 

 tially identical. 



Alimentary canal. — The mouth of Synaptids is a circular opening in the 

 center of the oral disc, ai-ound which the tentacles form a single circle, and 

 which constitutes the anterior end of the body. The mouth opens at once into 

 a slender thin-walled oesophagus of variable length, which is encircled by the 

 calcareous ring and water-vascular ring, and which is more or less connected 

 with them and with the body-wall by strands of connective tissue. The 

 oesophagus opens into a more or less well-marked stomach, the thicker and 

 more miascular walls of which usually distinguish it from both oesophagus and 

 intestine. The latter is usually thin-walled and of nearly uniform diameter; 

 it may be short and without a looj) (a condition practically unknown among 

 other holothurians) or it may be elongated to such an extent that from the 

 point of union with the stomach it bends forward and runs for a greater or less 

 distance toward the mouth before it bends again and takes its course back- 

 ward to the vent. The latter is the ordinary arrangement. It terminates in 

 a slight enlargement homologous with the cloaca of the lung-bearing holo- 

 thurians, but as no other organs enter it, in the Synaptids it is better called 

 the rectum. The vent or anal opening is always terminal. A cross-section 

 through the alimentary tract at any point reveals an outer epithelium, the 

 ordinary lining of the body-cavity; inside of this a very thin layer of connect- 

 ive tissue, following which is a muscular layer consisting first of longitudinal 

 and then of circular fibers ; within this is a thick layer of connective tissue, 

 and then the lining epithelium of the canal. The layer of muscles is thickest 

 on the stomach, the walls of which are decidedly muscular. The inner epithe- 

 lium is more or less glandular, especially in the stomach, and is frequently 

 ciliated, at least in the intestine. Some writers have described a delicate cuti- 

 cle there instead, but this is possibly a misinterpretation of the appearance 

 of the cilia in preserved material. Wandering cells (often bright red in the 

 living animal) occur more or less abundantly throughout the lining epithe- 

 lium. The alimentary canal is supported through part or all of its course by 

 a mesentery, usually consisting of three sections. The first supports the 

 oesophagus and stomach and is attached to the body-wall in the mid-dorsal 

 interradius; the second supports that part of the intestine which runs forward 

 from the end of the stomach and is attached in the left dorsal interradius ; the 

 thii-d supports the remainder of the intestine and is attached in the right 

 ventral interradius. The second section may be greatly reduced and is prac- 

 tically wanting in those species which ha\'« an approximately straight aliment- 

 arv canal. Tn other cases the first and second sections of the mesenterv run 



