440 The Animal Kingdom 



outer body wall is a syncytium, that is, there are many nuclei but no 

 dividing cell walls. One feature of the rotifers which is also found in 

 the members of other related phyla is that of cell constancy. In early 

 embryology, the number of cells present in each organ system is es- 

 tablished. This number does not change; instead, they fuse into a 

 syncytium with a definite number of nuclei. For a given species, the 

 number of nuclei is constant for the different organs. 



One form commonly seen creeping leechlike through protozoan 

 cultures is Philodina sp. This animal's body is seen to be divided into 

 three regions : an anterior head, a large trunk, and a posterior joot. 

 The head consists of a disc which is rimmed with cilia. These cilia 

 move rapidly and in this species look like two small wheels at the 

 anterior end. These cilia carry currents of water with their food and 

 oxygen to the animal and also aid in locomotion. The foot which 

 contains cement glands terminates in two spurs and several retractile 

 toes. The secretion from the cement glands helps to attach the ani- 

 mal temporarily to a substrate. Internally the body cavity is a pseiido- 

 coel containing the visceral organs. The digestive tract begins in the 

 mouth which is just below the corona. The mouth opens into the 

 pharynx which has a very muscular portion known as the mastax. In 

 this mastax are several chitinized jaws with teeth which are used to 

 break up the food. The pharynx opens into a short esophagus, and this 

 in turn opens into a heavy-walled stomach. The stomach opens into a 

 short intestine which terminates in a cloaca with an anus. Associated 

 with the stomach are digestive glands. 



The excretory system consists of two long protonephridial tubules 

 which empty into the cloaca. Each tubule is long and coiled and has 

 many side branches. These lateral branches each bear flame cells. 

 In function it appears that the excretory system is chiefly hydrostatic. 

 There is a large nerve ganglion or brain at the anterior end of the 

 body just dorsal to the mouth. From this brain various nerves go to 

 the different parts of the body. Some nerves connect to sensory 

 tufts of hair as well as to small dorsal eyespots. Long muscle fibers 

 are found in the body cavity. These are isolated and of the longitudinal 

 type. Their function is to retract the corona and to move the foot. 



The female has two ovaries and yolk glands. These open by 

 means of oviducts into the cloaca. Males have never been observed 

 in Philodina; however, in other species of rotifers, males are known. 

 In these, the male is very small and has one large testis in the body 



