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 Chcetopoda, which present the two phases. Though it is 



/68 



o 

 



o 



166 



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 : &quot; Le Proglottis 

 (segment) ayant acquis tout son developpement, se detache ordinairement de 

 la colonie et continue encore a croitre dans 1 intestin du memo animal ; il 

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

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

 des muscosites intestinales.&quot; 



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

 vidual CtenodriJus monostylus, which multiplies by dividing and subdividing 

 itself, &quot; parts arise which are destitute of both head and anus and at times 

 consist of only a single segment.&quot; In another species, C. pardalis, 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). 



