FORMATION OF THK WATER-TUBES. 13 



ing [a), which performed at first the functions of the moutli, is here.ifter 

 the anus (a) ; the second opening, the true mouth (?«), is not formed until 

 the embryo has arrived near the end of the second day ; it is placed in 

 the middle of the lower surface, and from this time forward the former 

 mouth assumes the function of an anus. That portion of the digestive 

 cavity which leads from the mouth to its bulging portion is the oesopha- 

 gus (o), the bulging portion is the true digestive cavity, or stomach proper 

 (d), the short tube leading from the stomach to the anus is the intestine 

 (c), while the diverticula (tv, tv) are the two branches of the future water- 

 system. The reasons for calling these parts mouth, anus, oesophagus, 

 stomach, intestine, and water-system will become apparent as we trace 

 the development of the embryo in its more advanced stages, in the fol- 

 lowing pages.* 



The currents, which before had entered through the mouth («), passed 

 to the extremity of the cavity (a), and been expelled again through the 

 same opening (a), now change their course completely ; there is a current 

 which enters the mouth (m), flows through the oesophagus (o) into the 

 diverticula {tu, tv), then into the true stomach [d], and is finally rejected 

 through the anus («). From this time forward it is quite an easy thing 

 to observe the course of the food ; it is taken into the mouth by means 

 of the currents produced around its opening, passes rapidly through the 

 oesophagus, rotates for some time in the spherical stomach (d), and then 

 passes out slowly through the opening (a) of the alimentary canal (c). 

 As these currents are more and more distinct as the larvae grow older, 

 there can be no doubt that the function of the first-formed opening is 

 eventually confined to that of an anus, after having performed the func- 

 tion of mouth during the first stage of growth of the larva. 



Formation of ihe Watcr-Tvbes. — By w\ater-tubes I mean the bodies which 

 have received from Miiller the name of problematic bodies, in their earlier 

 stages of growth, and which he has called Schlauchsystem, when they 

 appear, in the older larvoe, as broad tubes running on each side of the 

 oesophagus and stomach. These parts he considered as independent sys- 

 tems, but as they are only different stages of the same thing, as will 

 appear below, they have received here the name which denotes most 



* Other terms arc also frequently used, to denote the different parts of radiated animals, which are 

 not usually adopted ; they will be found fully explained in the thi'-d volume of the Contributions to 

 the Natural History of the United States, by I'rof. Agissiz, p. 73, and seq. 



