THE PROTOZOA-THE DAWN OF LIFE 21 



Extremely abundant round our English coasts, and one of the 

 chief causes of the phosphorescence of the sea on summer nights, 

 Noctiluca miliaris is a familiar object to most students of the 

 microscope. It belongs to the sub-class Cystoflagellata, and has 

 a very wide geographical distribution. Noctiluca is quite a giant 

 Flagellata, measuring about I mm. or more in diameter. In shape 

 it is something like a peach, having a similar indentation or cleft 

 on one side, from which arises the large, stout, striated flagellum. 

 Two lip-like prominences may be seen in front of the base of this 

 large flagellum ; one, transversely ridged, and slightly firmer 

 than the other, is called the " tooth." At the junction of these 

 two prominences is a second and minute flagellum, the cilium, 

 and behind these is the oval mouth, through which food is absorbed. 

 The phosphorescence glows with a bluish or greenish light when 

 the little Noctiluca is disturbed, disappearing when the cause 

 of irritation ceases. 



One large group of Flagellates inhabiting both fresh and 

 salt waters, and called Collared Monads (Choanoflagellata) are of 

 particular interest as probably representing a link or transition 

 towards the sponges. The members of this group are charac- 

 terised by a remarkable outgrowth of the protoplasm around 

 the base of the single flagellum, giving to that organ the appear- 

 ance of being surrounded by a transparent collar. This collar 

 is contractile, and its movements appear to produce currents in 

 the water which carry the organic particles of matter upon which 

 the monad feeds down to the soft protoplasm between the base 

 of the flagellum and that of the collar, when a food-vacuole is 

 formed for their ingestion. The ordinary mode of multiplication 

 among these Collared Monads is by longitudinal fission extending 

 up through the funnel or collar. 



Belonging to this group is a colonial form called Protero- 

 spongia, which secretes a gelatinous investment by which it be- 

 comes attached to solid bodies. According to the observations 

 of the late Mr. Saville Kent, the central members of the colony 

 retract their collar, lose their flagellum, become amoeboid, finally 

 undergoing brood-formation to produce minute zoospores, com- 

 parable to the spermatozoa of a sponge. A colony of Proterospongia 

 strongly recalls many of the characteristics of the true sponges, 

 and indeed has been regarded as a transition towards them, for 



