In Lumbricus and Criodrilus the blastopore remains as the mouth, but in Euaxes a new mouth or rather stomodæum is formed by an epiblastic invagination between the front end of the two mesoblastic bands. This epiblastic invagination forms the permanent oesophagus; and in Lumbricus trapezoides and Criodrilus, where the oral opening is at first lined by hypoblast, the epiblast soon becomes inflected so as to line the oesophageal region. The splanchnic mesoblast of the cephalic region subsequently invests the oesophagus, and some of its cells penetrating between the adjoining epiblast cells give rise to a thick wall for this part of the alimentary tract; the original epiblast cells being reduced to a thin membrane. This mesoblastic wall is sharply separated from the muscular wall outside, which is also formed of splanchnic mesoblast. The anus is a late formation. _Alternations of generations._ Amongst Chætopoda a considerable number of forms exhibit the phenomenon of alternations of generations, which in the same general way as in the case of the Coelenterata, is secondarily caused by budding or fission. The process of fission essentially consists in the division of a parent form into two zooids by the formation of a zone of fission between two old rings, which becomes differentiated (1) into an anal zone in front which forms the anal region of the anterior zooid, and (2) into a cephalic zone behind which forms the head and some of the succeeding segments of the posterior zooid. The anal zone is capable, by growth and successive segmentation, of giving rise to an indefinite number of fresh segments. In Protula Dysteri, as shewn by Huxley, there is a simple fission into two in the way described. Sexual reproduction does not take place at the same time as reproduction by fission, but both zooids produced are quite similar and multiply sexually. In the freshwater forms Nais and Chætogaster a more or less similar phenomenon takes place. By a continual process of growth in the anal zones, and the formation of fresh zones of fission whenever four or five segments are added in front of an anal zone, complicated chains of adhering zooids are produced, each with a small number of segments. As long as the process of fission continues sexual products are not developed, but eventually the chains break up, the individuals derived from them cease to go on budding, and, after developing a considerably greater number of segments than in the asexual state, reproduce themselves sexually. The forms developed from the ovum then repeat again the phenomenon of budding, etc., and so the cycle is continued[142]. [142] Reproduction by budding and formation of the sexual products to some extent overlap.