6io 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



Larvae of Podophrya fixa are more or less ovoid in form and in trans- 

 verse section, and cilia are present over the whole surface. Before attach- 

 ment they become spherical and the cilia undergo regression, an equa- 

 torial band persisting longest. Another larval type is more or less dis- 

 coid, circular in outline, with disk of attachment in the center of one 

 flattened face and a marginal band of cilia (Fig. 192, A). This form may 

 be regarded as possessing a polar pattern vertical to the plane of flatten- 

 ing and as completely radial. A modification of this type appears in larvae 



B 



Fig. 192, A-D. — Larval forms of Suctoria. A, Discophrya cybislri, optical section, showing 

 granules about disk of attachment, meganucleus, two vacuoles, and ciliary band; B, Toko- 

 pkrya cyclopum, mature attached stage; C, "larval" stage of B, showing disk of attachment 

 with granules, cilia, meganucleus, and one vacuole; D, Acineta tuberosa, with chief a.xis 

 obliquely anteroposterior (after Collin, 191 2). 



elongated vertically to this polar axis and therefore possessing a physio- 

 logical anteroposterior pattern and a bilaterality, the original polar axis 

 being physiologically dorsiventral with the disk of attachment ventral. 

 According to Collin, the physiologically posterior end of the larva is the 

 end of the bud last detached from the parent. These larvae usually move 

 with the "ventral" surface in contact with a substrate. In larvae of certain 

 other species the disk of attachment is at one pole of a longitudinal axis, 

 and this pole is anterior in locomotion. These forms possess an equatorial 

 ciliary band consisting of rows of cilia, different in number in different 

 species, and a group of cilia at one side of the pole opposite the disk of 



