ii PHYLUM PROTOZOA 101 



individuals by a process of multiple fission. After complicated 

 changes in the nuclei of the two conjugating individuals, during 

 which the microgamete contributes a micronucleus towards the 

 formation of the new nuclear apparatus of the megagamete, the 

 former shrivels and dies (G 1 , G 2 ). The essence of conjugation is 

 the reception of nuclear material derived from another individual : 

 its effect appears to be a renewal of vitality, usually manifesting itself 

 in increased activity in multiplication by fission. 



Though most Ciliata are free, many are parasites, mainly in the 

 alimentary canal of various Metazoa. Pathogenic forms are almost 

 unknown. 



ORDER 2. TENTACULIFERA. 



Judged from the adult structure alone, the members of this 

 order would certainly be placed in a separate class of the Protozoa : 

 it is only in virtue of the facts of development that they are 

 united in a single class with the Ciliata. 



The body may be globular (Fig. 77, la), ovoid (76), or cup- 

 shaped (?a), but presents nothing like the variety of form met 

 with among the Ciliata. The distinguishing feature of the group 

 is furnished by the tentacles which are always present in greater 

 or less number, and which, in some cases at least, are the most 

 highly differentiated organs found in the whole group of Protozoa. 

 The characters of the tentacles vary strikingly in the different 



genera. 



In the common forms Acineta (2), and Podoplirya (1), the ten- 

 tacles spring either from the whole surface, or in groups from the 

 angles of the somewhat triangular body. Each tentacle is an 

 elongated cylindrical structure (Ic), capable of protrusion and 

 retraction, and having its distal end sucker-like. It is, moreover, 

 practically tubular, the axial region consisting of a semi-fluid 

 protoplasm, while the outer portion is tolerably firm and resistant. 

 When partially retracted, a spiral ridge is sometimes observable 

 around the tentacle : this may indicate the presence of a band of 

 specially contractile protoplasm, resembling the axial fibre in the 

 stalk of Vorticella. Infusors and other organisms are caught by 

 the tentacles (4, 6), the cuticle of the prey is pierced or dissolved 

 where the sucker touches it, and the semi-fluid protoplasm can 

 then be seen flowing down the tentacle into the body of the 

 captor. A single tentacle only may be present (3), or the tentacle 

 may be branched (4], the extremity of each branch being suc- 

 torial. In some forms there are no terminal suckers (5), and the 

 tentacles are waved about to catch the prey instead of standing 

 out stiffly as in Acineta. In other cases there are one or more 

 long, striated tentacles with tufted ends (7). 



The nucleus may be ovoid (la), horseshoe-shaped, or branched 

 (S, 9) : in many cases a micronucleus (la, mi. nu.) has been found, 



