Chap. I. FORMATION OF THE WATER-TUBES. 11 



from above (PI. II. Fig. 8), at right angles from the main cavity, but trend obliquely 

 upwards, as seen in profile (PL II. Fig. 7), towards the other extremity of the embryo, 

 as in Figs. 7, 10, PI. II. The outer wall, which had formed a connection with the closed 

 extremity of the digestive cavity, on the lower side, has been drawn out in the shape 

 of a slender cone (o, PL II. Figs. 7, 10, 11, 14, 17), and becomes the oesophagus, 

 which leads to an opening (m, the mouth), connecting the ventral side with the 

 digestive cavity. 



Nomenclature. — It will materially assist in the explanation of the subsequent 

 changes of form, and obviate a great deal of circumlocution, if we at once call the 

 different organs by their true names. The original opening (a), which performed 

 at first the functions of the mouth, is hereafter the anus (a) ; the second open- 

 ing, the true mouth (in), 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 

 oesophagus (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), wdiile the 

 diverticula (w, to) 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 following pages. 1 



The currents, which before had entered through the mouth (a), gone to the extrem- 

 ity of the cavity (a), and been expelled again through the same opening («), now 

 change their course completely ; there is a current which enters the mouth (m), 

 passes through the oesophagus (o) into the diverticula (?c, tv'), then into the true 

 stomach (d), and is finally rejected through the anus (a). 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 (e). As these currents are 

 more and more distinct as the larvae grow older, there can be no doubt that the func- 

 tion of the first-formed opening is eventually confined to that of an anus, after having 

 performed the function of mouth during the first stage of growth of the larva. 



Formation of the Water -Tubes. — By water- tubes, I mean the bodies which have 

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



1 Oilier terms are also frequently used, to denote in the third volume of the Contributions to the Natu- 

 the different parts of radiated animals, which are not ral History of the United States, by Prof. Agassiz, 

 usually adopted ; they will be found fully explained p. 73, and seq. 



