THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 103 



represented in Fig. 167, we have a species in which the 

 number of segments thus united does not exceed four. In 

 Echinobothrium typus there are eight or ten ; and in cestoids 

 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 Platyhelminthes and 

 the Chwtopoda, 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, 



* 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 supposed 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 l'intestin du meme animal ; il 

 change meme souvent de forme et semble doue' d'une nouvelle vie ; ses angles 

 s'effacent, tout le corps s'arrondit, et il nage comme une Planaire au milieu 

 des muscosites intestinales." 



f Though this was doubtful in 1865 it is no longer doubtful. In an indi- 

 vidual Ctenodri/us movostylus, which multiplies by dividing and subdividing 

 itself, " parts arise which are destitute of both head and anus and at times 

 consist of only a single segment." In another species, C. partialis, there is 

 separation into many segments ; and each segment before separating forms 

 a budding zone out of which other segments are afterwards produced, com- 

 pleting the animal (Korschelt and Heider, Embryology, i, 301-2). 



