MASTIGOPHORA: CHOANOFLAGELLATA. 847 



differing intensities and perhaps also at different stages of their life. Small 

 as they are, Flagellata are not exempt from the attacks of parasites, 

 e. g. the colonies of Volvox afford shelter and food to two species of the 

 Rotiferan Notommata and to the Proteomyxan Pseudospora ; and many of 

 them are infested by species of the fungus Chytridium, which live and 

 multiply within them ; hence erroneous views as to their reproduction. 



(ii.) Choanoflagellata (Craspedomonadina, s. Cyclomastiges). This sub-class, 

 found in both fresh and salt water attached to various objects, especially 

 the stalks of Vorticellids, is distinguished by two special features : (i) the 

 possession of a single fine flagellum, implanted at one extremity of the 

 body, within an area surrounded by (2) a clear funnel-shaped collar of 

 protoplasm. 



The body is ovate, contractile, and sometimes very liable to assume 

 an amoeboid condition (Proterospongia) ; it is minute, and never exceeds 

 Tinnr in. in size, of a bluish tint, and consists of a colourless protoplasm 

 charged with more or fewer granules. The collar is expansile and re- 

 tractile, either short, narrow, and smaller at its aperture than at its base 

 (Phalansterina), or as is more general, the reverse (Craspedomonadina). It is 

 extremely sensitive to movements in the water. It appears functionally 

 to be connected with nutrition (Pin Phalansterina). The motion of the 

 flagellum brings floating particles from behind forwards, and when they 

 impinge against the collar they adhere to its outer surface. They are 

 then either carried by a flow of protoplasm upward to its edge, over it, 

 and downwards to the central area where they are engulfed (Kent) ; or 

 the direction of the flow is the reverse, and the particles when they reach 

 the base of the collar are either taken up by a vacuole formed temporarily 

 (Biitschli) or carried into the body by a separation of a portion of the 

 protoplasm of the collar (Entz). The food may be lodged within food 

 vacuoles, and consists principally of Bacteria. Faecal residues are ex- 

 pelled within the area surrounded by the collar. The nucleus is small, 

 round, vesicular, placed near the base of the flagellum. Contractile 

 vacuoles are always present at the base of the body, usually two in number, 

 but as many as five have been seen. 



The animal may be sessile or stalked, and in the latter case the stalk 

 may be simple and support a single individual, or carry a number of 

 individuals attached to its apex by short contractile threads (Codosigd) 

 or it may be branched (Codonocladium). The simple forms are generally 

 social. Hirmidium (=Desmarelld) occurs in floating colonies in which the 

 individuals are united side by side in band-like rows, or irregularly ; Astro- 

 siga is similarly free, but the members of the colony are united by their 

 posterior extremities into stellate clusters. Some are furnished with a 

 gelatinous investment, either clear (Proterospongia) or somewhat granular, 



