112 PROCEEDINGS OF THE AMERICAN ACADEMY 



Fig. 8. An embryo somewhat more advanced than Fig. 6, seen in profile, to 

 show that this digestive cavity does not remain in the centre of the body, 

 but bends towards one side, and eventually joins the wall of the lower side 

 as seen here (m). This point of junction is the indication of the future 

 mouth, after the formation of which the first mouth assumes exclusively 

 the functions of an anus, which is also now bent down, while the anal 

 extremity of the embryo («) is slightly bevelled. 



Fig. 9. An embryo of Asteracanthion herylinus still more advanced than Fig. 8, 

 seen in profile ; at this time the digestive cavity has been diiferentiated into 

 three distinct parts, — a retort-shaped stomach, of which the bulb is the 

 stomach (rZ), and the tube the alimentary canal, the walls of which are quite 

 thick, while the walls of the oesophagus are quite thin and transparent, 

 capable at the same time of great contraction and expansion. The mouth 

 (jri) is seen like a large opening in the middle of the depression on the 

 lower side, leading into a pistol-shaped oesophagus. Immediately over this 

 is seen one of the water-tubes communicating by means of a tube (if') 

 ■with the outer medium. This opening (&) becomes eventually the madre- 

 poric body. On the opposite side there is a similar water-tube which has 

 no connection with this one, and does not open outward. The two small 

 protuberances (v, v) on the sides of the mouth are the first signs of the 

 chord of vibratile cilia. 



Figs. 10-18. — Asteracanthion pallidus. 



Fig. 10. A young embryo of Asteracanthion pallklus seen fi'om the mouth side, 

 to show the position of the two water-tubes {lo, tu'), and also the manner 

 in which the vibratile ciHa commence as two independent arcs on ^each 

 side of the mouth. The broad oesophagus is also seen leading into the 

 round stomach (fZ), which empties through a narrow alimentary canal (c), 

 through an anus (a) placed near the upper arc of vibratile cilia. 



Fig. 1 1 . An embryo in which all the parts of the Brachiolaria can already 

 be detected. (Compare with Fig. 12.) The two simple arcs of vibratile cilia 

 have formed two independent plastrons, the corners and indentations of 

 which are the first ti'aces of the future arms (e', e", e'", e""). The two 

 independent water-tubes of the preceding figure have increased in size ; 

 they now extend beyond the level of the mouth, and form a broad Y- 

 shaped body, which surrounds the oesophagus (o) and extends from the 

 mouth to the opposite extremity of the stomach. 



Fig. 12. A Brachiolaria in which the corners of the vibratile chord of the 

 embryo of the preceding figure have developed into long arms (e', e", e" . . .). 

 There are also three small heavy appendages {/■,/")■, surmounted by short 

 ■warts, which did not exist in the other figures, situated at the extremity 

 opposite to that where the young echinoderm is developed. Branches of 

 the water system (/') enter into these arms, as shown In the following 



