126 MARINE ANIMALS OF MASSACHUSETTS BAT. 



shall see, remain separate, and fonn an open angle with one 

 another ; it is only toward the end of the development that they 

 unite, enclosing between them the internal organs, which hare 

 been built up in the mean while. 



At about the same time with the development' of these two 

 pouches, so important in the animal's future history, the digestive 

 cavity becomes slightly curved, bending its upper end sideways 



till it meets the outer wall, and forms 

 a junction with it (m. Fig. 164). At 

 this point, when the juncture takes 

 place, an aperture is presently formed, 

 which is the true mouth. The diges- 

 tive sac, which has thiis far served as 

 the only internal cavity, now contracts 

 at certain distances, and forms three 

 distinct, though connected cavities, as 

 in Fig. 163 ; viz. the oesophagus lead- 

 ing directly from the mouth (m) to the 

 second cavity or stomach (<^), which 

 opens in its turn into the third cavity, 

 the alimentary canal. Meanwhile the water-tubes have been 

 elongating till they now surround the digestive cavity, extending 

 on the other side of it beyond the mouth, where they unite, thus 

 forming a Y-shaped tube, narrowing at one extremity, and divid- 

 ing into two branches toward the other end. (Fig. 165.) 



On the surface where the mouth is formed, and very near it on 

 either side, two small arcs arise, as v in Fig. 162 ; these are cords 

 consisting entirely of vibratile cilia. They are the locomotive 

 organs of the young embryo, and they gradually extend until 

 they respectively enclose nearly the whole of the upper and lower 

 half of the body, forming two large shields or plastrons. (Figs. 

 165, 166.) The corners of these shields project, slightly at first 

 (Fig. 165), but elongating more and more until a number of 

 arms are formed, stretchuig in various directions (Figs. 166, 167), 

 and, by their constant upward and downward play, moving the 

 embryo about in the water. 



Pig. 166. Larva In which arras are developing, lettering as before ; e' c'' e"' e* eS efl ariuB, o oeiopb. 



IgOB 



