28 Invertebrata. 



sometimes form masses of more than an inch in dia- 

 meter, which are often found floating on standing water. 

 The commonest forms of Infusoria are the slipper 

 animalcule (fig. 14), the boat-like animalcule or u- 

 plotes (fig. i), and the hay-infusion animal or Colpoda, 

 but almost every infusion has its own form of animal. 



Several groups of microscopic animals are allied to 

 the Infusoria. Some of these are called monads and 

 are mouthless nucleated bodies with one long cilium. 

 Another of these is Noctiluca (fig. 15), a globular 

 creature about -^th of an inch long, with a short obtuse 

 vibrating flagellum or filament and a mouth, but 

 whose interior consists of netted protoplasmic threads 

 whose meshes are filled with water. These organisms 

 are among the commonest of those to which the sea 

 owes its phosphorescence. 



Other minute forms, called Acinetse (fig. 16), are 

 small, stalked masses whose surface is studded with 

 radiating, retractile tubular suckers, through which 

 they suck the juices of their prey, 



CHAPTER VI. 



SUB-KINGDOM II. SPONGES (POLYSTOMATA). 



Metazoa, All animals above the Protozoa possess 

 an internal body-cavity, the wall around which is 

 made up of two primary layers, often with difficulty 

 discriminate in the lowest forms ; and there is either 

 one terminal mouth into the cavity, or, as in the case of 

 the sponges, many lateral pores communicate there- 

 with. 



