THE INFUSORIA 



195 



The larger form or macrogamete is usually a normal-sized individual, 

 although in some cases it is somewhat larger than the ordinary cells 

 (Zoothamnium). The microgametes, on the other hand, are con- 

 siderably smaller, and from four to eight are formed by each cell. 

 These never develop a stalk, but leave the parent colony, and swim 

 about by means of the ring of cilia around the lower pole. They 

 finally come in contact with the macrogamete and fuse with it, the 

 union taking place at the lower end of the attached organism, and 

 near the insertion point of the stalk (Fig. 107). 



II. THE SUCTORIA 



The Suctoria differ decidedly from the Ciliata, from which they 

 have undoubtedly sprung. With the exception of Hypocoma (Fig. 

 115, C), which remains ciliated throughout life, the Suctoria possess 

 cilia only during the embryonic stages. They are, for the most part, 

 sedentary forms, and grow upon a chitinous peduncle, which is at- 

 tached at the lower end to some foreign object. The upper end 

 of the peduncle is hollowed out into a bowl, within which the animal 

 lies. Owing to its attached mode of life, and to the equal pressure 

 on all sides, the general form of the animal is spherical or radially 

 symmetrical. In some cases there is a well-defined membrane, but 

 the various students of the group are not agreed as to its structure. 

 It is never striated, as in the Ciliata, and there is no cortical plasm. 

 The endoplasm shows no differentiations other than the usual food 

 granules or assimilation products common to all Protozoa. 



An essential point of difference from the ciliate structure is the 

 presence of tentacles, which, in the majority of Suctoria, are the only 

 motile appendages of the adult. In many respects they are similar 

 to the tentacles of Actinobolus, Ileonema, and Mesodinium (Fig. 115), 

 but differ in the very important fact that they are hollow, while the 

 extremities bear the mouth openings. 



There are two general types of tentacles : one, according to Biit- 

 schli, captures prey, while the other devours it. Of the latter forms, 

 there are also two types. One is long and broad, and, like a thorn, 

 pointed at the extremity ; the other is nearly uniform in diameter and 

 flattened at the top, or hollowed out into a cup-like sucking organ 

 (Fig. 108). These are distinguished as the styliform and capitate 

 tentacles (Delage). Both sets of tentacles are hollow, and their 

 lumena open at the ends. There is a difference of opinion, however, 

 in regard to their inner structure and function. Biitschli holds that 

 some of them are solid, and others hollow. He maintains that, in 

 the solid forms, the internal portion is formed of endoplasm, which is 

 continuous with the inner plasm of the cell. Delage claims that they 



