68 E. A. ANDREWS. 



passes readily back and forth from more complex sedentary to 

 more simplified free-swimming phases of form and activity. 



In the sedentary phase the animal occupies a house or case 

 from which it may protrude two very long ligulate lobes that 

 make its feeding apparatus so effective. In this phase the 

 animal presents the maximum differentiation and is capable of 

 division, as described in part by Mobius (9) and more in detail 

 by Sahrlage (4). This is the adult or perfect phase. In the 

 free-swimming phase the animal lacks the feeding lobes, the 

 mouth and the special organ for attachment and it is in fact 

 a simplified, retrograded or dedifferentiated form which some 

 have called a larval form. It is, however, the free-swimming 

 form that fabricates the house which it will occupy when it has 

 differentiated into the perfect form, from which state it may again 

 retrograde back into the simplified free-swimming stage and 

 again leave its house. 



Briefly, the general structure of these two forms is as follows : 

 The free-swimmer is cylindrical, capable of great extension and 

 contraction with spiral and lateral bendings. It moves by means 

 of about 100 lengthwise lines of cilia. At the posterior end the 

 cilia are absent over an area within which is a complex, problem- 

 atical organ into which the myonemes converge. At the anterior 

 end there is a very small, nearly circular spiral of membranells 

 which though active seem to have no special use. Where the 

 infundibulum and mouth might be expected there is a small 

 microscopic vestige of no conceivable use. The animal captures 

 no food. The food that may be inside of it was obtained in the 

 previous phase when the feeding lobes and mouth were present. 

 Essentially this free-swimmer is the adult broken loose from its 

 former attachment to the bottom of its house or case and de- 

 differentiated to the extent of entire resolution of its lobes, 

 pharynx, and hypostome. The only vestige of all its former 

 feeding apparatus being the above-mentioned minute ectosarcal 

 cup and the single limb of a spiral hypostome. 



On the other hand the anatomy of the adult sedentary form is 

 more complicated. In addition to the long lines of cilia and 

 pigment underlaid by very highly specialized myonemes, there 

 is at the posterior or foot end a special organ of fixation to the 



