4 MORPHOLOGY OF INVERTEBRATE TYPES 



cortical layer. The pellicula is a thin elastic membrane covering 

 the entire body. It has a distinctly striated appearance due to 

 the fact that the surface is divided into small hexagonal fields 

 separated from each other by ridges. From the center of each 

 field arises a long cilium. Under the base of the cilium is a 

 motor center in the shape of a small grain from which a fibre 

 runs toward the endoplasm. The rhythmic beating of the cilia 

 produces a pressure on the water and propels the animal in a 

 wide spiral course. At the posterior end of the body are several 

 immobile cilia probably with tactile function. In the angles 

 formed by the ridges of the pellicula as well as in the middle of 

 some ridges are the insertion points of the trichocyst ends. 

 The alveolar layer is situated immediately under the pellicula 

 and consists of a single layer of alveoli with walls at right angles 

 to the surface. The innermost layer of the ectoplasm is the 

 cortical layer which contains the trichocysts and the pulsating 

 vacuols. The trichocysts are protective organella. They are 

 present in immense numbers and have the shape of small spindles 

 with a thickened end directed toward the surface and a rod-like 

 process inserted in the ridges of the pellicula, as described above. 

 When the animal is irritated the trichocysts " explode" and 

 appear as long and thin threads acting as harpoons. The Pul- 

 sating or contractile vacuoles are excretory organella. They con- 

 sist of a central collecting vacuole and from seven to ten afferent 

 vacuoles. The central vacuole is distended to its limit when the 

 afferent vacuoles are contracted and is in systole when the 

 afferent vacuoles are in diastole. The afferent vacuoles empty 

 their content into the central vacuole which in its turn empties 

 the fluid to the outside through a small excretory pore situated in 

 the pellicula over the center of the vacuole. 



We have seen already that the mouth or cytostome is situated 

 at the bottom of the peristome and leads into a short, curved 

 cytopharynx. The undulating membrane which is attached to the 

 wall of the cytopharynx is in continuous motion, drawing food 

 particles toward the posterior end of the cytopharynx where the 



