170 THE EVOLUTION OF THE METAZOA 



attached to the bottom. In the first case, the evolution has fol- 

 lowed a somewhat progressive line even if in a different direction 

 to the one that can be observed in Turbellaria. In the second 

 case, there was a strong retrogression which finally led to a 

 complete loss of the gullet. The form of the gullet tube changed 

 in accordance with the change of the form of the mouth (it is only 

 rarely that the round opening has been preserved) into an oblong 

 or oval opening which can be laterally or transversally closed. 

 It is not easy to determine which corner of the mouth is dorsal 

 and which is ventral. The two corners frequently continue in 

 a long tip w^hich extends deep into the interior of the body. 

 A well-known groove, the siphonoglyph or the sulcus, which 

 is densely covered with cilia, develops along one corner of the 

 mouth (called the "ventral" corner), and frequently also along 

 the two corners of the oral opening. Furthermore, a rich long- 

 itudinal folding can appear in this oral tube, a sphincter at 

 its lower end, and special, even if poorly developed, muscles 

 which usually extend in the longitudinal direction, a so-called 

 concha, etc. Polymerization can be observed, surprisingly 

 enough, in siphonoglyphs, this reminds us at least partly of the 

 polymerization of whole pharynxes of some planarian species. 

 A further complication appears due to the linkage of the 

 gullet tube with the proximal parts of sarcosepta. Because of 

 this, the gullet tube can reach deeply into the interior of the 

 animal's body. Parallel to the morphological changes, which 

 have led to a further specialization, physiological changes 

 also took place (e.g. respiration by means of a circulation 

 of water). 



A reduction of the gullet tube appears as early as in Antho- 

 zoa; it is frequently a consequence of the formation of cormi 

 and, in connection with this, of a considerable diminution of 

 subindividuals, combined with microphagy. The gullet tube 

 becomes shorter and shorter, the siphonoglyphs and muscles 

 disappear, and finally there develops an oral cone which 

 surmounts the oral surface, as can be observed in some 

 Madreporaria of the solitary species Balanophjlia. In this way 



