156 



EMBRYOLOGY 



ing upon ciliary motion here functions as the principal 

 locomotor apparatus. This primitive form of motion ac- 

 quires in this case an importance and a development such 

 as exists nowhere else in the animal kingdom, whereas in 

 the Cnidaria it is not so prominent. The presence of the 

 sensory organ at the apical pole, which is, perhaps, to be 

 explained as tlie central point of the nervous system, makes 

 it seem unlikely that in any ancestral form there existed 

 at this point a region of separation, a cicatrized place of 

 attachment. Furthermore the abundance of organs on the 

 outer surface of the body (which would correspond to the 

 ex-umbrella) argues against direct relationships between 

 Medusae and Ctenophora. 



From what precedes we must conclude as most probable 

 that the Ctenophora represent an independent stem of the 

 animal kingdom, which is connected with the Cnidaria 

 (Coelenterata in the restricted sense) only at its roots, and 

 has in common with them only those ancestral forms which 

 preceded the passage and metamorphosis into the polyp 

 form. The Ctenophora have most probably always retained 

 the original pelagic mode of life, and have brought to the 

 highest state of development the likewise primitive form of 

 motion by means of cilia, without exchanging it for the 

 secondary kind of movement by means of muscular action. 

 If we were to form a picture of the hypothetical pelagic 

 ancestral form of the Ctenophora, it would probably corre- 

 spond most nearly to certain Actinian larvce, which exhibit a 

 tuft of cilia at the anterior end and the mouth-opening at 

 the posterior pole, while within, the development of the 

 gastral pouches has already begun by the formation of 

 septa. The tuft of cilia at the anterior end of the body 

 would furnish the starting-point for the development of the 

 apical sensory organ, while the development of the ribs 

 would have advanced hand in hand with the further develop- 

 ment of the gastral pouches. 



If, then, we admit that the Ctenophora and Cnidaria have 

 a common stem only at their very beginnings, the question 

 arises how far the Ctenophora exhibit relationships to the 

 hypothetical ancestral form of the Bilateria. Apparently 



