THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 



generally they are numerous.* A considerable 



hiatus occurs between this phase of integration and the next 

 higher phase which we meet with; but it is not greater 

 than the hiatus between the types of the Annuloida and the 

 Annelida, which present the two phases. Though it is 

 doubtful whether separation of single segments occurs among 

 the Annelida, yet very often we find strings of segments, 

 arising by repeated longitudinal budding, which after reach- 

 ing certain lengths undergo spontaneous fission : in some 

 cases doing this so as to form two or more similar strings 

 of segments constituting independent individuals ; and in 

 other cases doing it so that the segments spontaneously 

 separated are but a small part of the string. Thus a Syllix, 

 Fig. 168, after reaching a certain length, begins to trans- 







166 



form itself into two individuals : one of the posterior seg- 

 ments develops into a head, and simultaneously narrows its 

 connexion with the preceding segments, from which it 



* I find that the reasons for regarding the segment of a Tcenia as answering 

 to an individual of the second order of aggregation, are much stronger than I sup- 

 posed when writing the above. Van Beneden says : " Le Proglottis (segment) 

 ayant acquis tout son developpement, se detache ordinairement de la colonie et 

 continue encore a croitre dans 1'intestin du mcme animal ; il change meme sou- 

 vent de form ft et semble doue d'une nouvelle vie ; ses angles s'effacent, tout le corps 

 s'arron.lit, et il na^e comme une Planaire au milieu des -nuscosites intestinales." 

 VOL. II. 5 



