Chap. I. FORMATION OF THE MOUTH. 9 



by step, the changes taking place in the pouch of the embryo, which is in this early 

 stage its digestive cavity (d) ; for it is a digestive cavity as much as the digestive 

 cavity of a young Actinia or a Scyphistoma, where the same opening serves as mouth 

 and anus. The mode of formation of the digestive cavity is entirely different in the 

 two classes ; in the Polyp it is hollowed out of the interior of the embryo, while in 

 the Echinoderm the bending in of the wall forms the stomach. Hence the two cavities 

 are not homologous, and the openings which lead into them, though performing sim- 

 ilar functions — those of mouth and anus — are likewise in no way homologous, though 

 they are in all built upon the plan of radiation. This opening always retains its 

 double function in the Polyps and some of the Acalephs, while in the Echinoderms it 

 becomes the anus after the true mouth has been formed, and the currents have 

 ceased to circulate in the extremity of the pouch and to pass out through the 

 same opening which admitted them. 



If there is any doubt that Echinoderms, Acalephs, and Polyps belong to the 

 same great type of ihe animal kingdom, a comparison of the young Echinoderm, 

 Acaleph, or Polyp in their earlier stages of growth, at a time when the spherosome 

 has not yet been divided into its component spheromeres, will show how great their 

 identity of development is, and how little there is in nature to base a separation into 

 Echinoderms and Ccelenterata, of this most natural great division of the animal 

 kingdom, the Radiates. I shall return to this point when speaking of the homologies 

 of the larvae of Echinoderms. 



Formation of the Mouth. — The perfect symmetry of the larva (PI. I. Fig. 27) is soon 

 modified, and in the next stages of development (PI. II. Figs. 2, 4), the digestive cavity 

 (d) no longer runs in the centre of the larva, but is bent slightly to one side. If 

 we examine one of the embryos about forty hours old (PI. II. Figs. 5, 6), we find 

 that great changes have taken place in the thickness of its walls. The outer wall 

 has everywhere become much thinner, except near the opening thus far called mouth, 

 where the decrease is not so marked. The walls of the digestive cavity, which were 

 of an equal thickness for the whole length, have become exceedingly attenuated at 

 the bottom of the sac, and have dilated to a considerable extent, forming a sort of 

 reservoir with very thin walls at the extremity of the pouch (PL II. Figs. 4, 6, d, 

 magnified and isolated, Fig. 1, d). These changes in the thickness of the walls, and 

 in the form of the internal cavity, are also accompanied by corresponding changes of 

 form in the embryo as a whole. The extremity opposite the so-called mouth has 

 increased in bulk, and greatly exceeds in size the perforated extremity (PI. II. Figs. 

 4, 6) of the body. 



When seen in profile (PI. II. Figs. 2, 4, 5), still greater changes are visible ; 

 there is a decided difference between the two sides of the embryo, forming what is 

 to become above and below; calling that part below, where the mouth is situated in 



vol. v. 2 



