848 THE ANIMAL KINGDOM. 



with the outlines of the constituent tubes visible, and either incrusting or 

 erect and branching (Phalansterium}. In the Salpingoecina each in- 

 dividual is lodged in a sessile or stalked theca ; it may be free, or attached 

 to the bottom of the theca by a delicate filament of protoplasm. The 

 theca itself is rarely thick and soft, usually thin, firm and colourless, ovate, 

 vasiform, horn-like, or, if it incloses the collar as well as the body, expanded 

 terminally. In Polyoeca the peduncle of one theca is attached to the 

 margin of another. The theca may be detached, and then the animal 

 swims about with it, the flagellum, as is always the case in a free Choano- 

 flagellate, pointing backwards in the line of motion. 



Reproduction is by binary fission, transverse in Phalansterium, and 

 most commonly in Salpingoeca, longitudinal in others, so far as is known. 

 In the former case the collar and flagellum are retracted, and the de- 

 tached portion may be amoebiform, but eventually becomes a free- 

 swimming monad with a single flagellum until it settles down and forms 

 a collar, &c. In the latter case, as observed in Codosiga botrytis, the 

 flagellum is withdrawn, the body divides as does the collar, after however 

 the protrusion of two flagella, one for each new individual. Fissiparous 

 individuals of large size have been noticed in Codosiga cymosa. The 

 occurrence of conjugation is uncertain. Retraction of the flagellum and 

 collar, assumption of a spherical shape and formation of a cyst membrane 

 has been observed, in many instances accompanied by division of the 

 contents of the cyst into two, or as a rule into a number of spherical 

 bodies which are set free as uniflagellate monads. After swimming about 

 the latter become fixed and develope a collar, &c. 



(iii.) Dinoflagellata (Cilioflagellata, Peridinea, Arthrodele Flagellata in 

 part). This sub-class is characterised by a more or less pronounced 

 bilateral aspect of body, coupled with a certain degree of asymmetry ; by 

 the presence, save in a few cases, of an envelope or cuticle of a substance 

 akin to cellulose, though not identical with it 1 ; by having two flagella 

 implanted close to one another, one directed parallel to the long axis 

 of the body, the other usually transverse to it. The majority of genera 

 are marine and widely distributed ; a few are marine and freshwater, 

 whilst one genus, the Peridinidan Hemidinium, is exclusively freshwater. 

 Marine forms are said to occur in Alpine lakes, but some doubt attaches 

 to the identification of the species. The sub-class is divisible into the 

 Adinida and Dinifera. 



In the Adinida the body is elongated, compressed laterally, and 

 covered by a porous bivalved cuticle ; the two flagella are implanted at 



1 According to Klebs the membrane in Hemidinium and Ghnodinium pulvisculus is formed of 

 a substance not akin to cellulose. 



