44 



LOWER INVERTEBRATES. 



slender stalk to some foreign support. At the end of the body farthest from the sup- 

 port, a band of cilia surrounds a flattened disc, at one side of which is the opening 

 dignified by the name of mouth. On careful examination it is seen that the band of 

 cilia does not form a complete circle, but rather a spiral, the inner end of which passes 

 into the mouth and down the tube known as. the oesophagus. The nucleus is sausage- 

 shaped, and in many forms is coiled in a spiral. The 

 stalk is also an interesting portion of the anatomy, as 

 within the external cuticle one may by careful examin- 

 ation see a core of contractile protoplasm, whose func- 

 tion will appear further on. 



Since Vorticella is one of the most common of the 

 Infusoria, it may be well to give here a slight account 

 of its method of life, as illustrative of the physiology of 

 the whole class. On placing a little powdered carmine 

 in the water in which the Vorticellce are living, and ex- 

 amining them under the microscope, it will be seen that 

 the motion of the ciUa around the mouth creates a cur- 

 rent of water, which pursues a constant direction down 

 the oesophagus and then out again. The particles of 

 carmine and other small bodies in the water follow the 

 general course of the current down to the bottom of the 

 oesophagus but are not allowed to go out again. In this 

 way a ball of nutritious matter is formed, which soon 

 forces its way into the protoplasm of the body, where it 

 is known as a food vacuole. These food vacuoles keep 

 up a constant though slow motion through the body, 

 passing down on one side and up on the other, until at 

 last all nutritious substance is digested, when the rejectamenta are forced out into the 

 beginning of the oesophagus and carried away from the body by the outgoing current 

 of water. 



It requires considerable time and much patient watching to make out these points 

 in VbrticeUa, for though attached, these animals are far from stationary ; every few mo- 

 ments the cilia wiU be suddenly withdrawn, and the animalcule wiU itself as suddenly dis- 

 appear. On moving the slide one readily ascertains the cause of this, for it will be seen 

 that the contractile protoplasm of the stem has exerted its powers, and the long, slender 

 stalk is now coiled in a close spiral. Gradually the stalk straightens out, when the 

 contractile vacuole renews its pulsations, and the cilia begin their vibration as suddenly 

 as they had stopped them a minute before. 



The process of binary division in these familiar objects is of high interest ; it is 

 longitudinal ; first the ciliary disc is vnthdrawn, and the body assumes a spherical con- 

 tour ; it soon becomes dilated, and a notch appears in the anterior border ; a new 

 vestibular cleft and oral system is developed on each side of the median line ; a line of 

 division now proceeds fi-om the anterior notch, through the centre of the animal's body, 

 cleaving both the contractile vesicle and nucleus. The result is two animalcules on a 

 single stalk. One zooid remains attached to the original pedicle, the other, with its 

 peristome usually contracted, develops round the posterior region of the body a circle of 

 cilia, by the action of which its attachment to the pedicle is broken, and it swims away^ 

 soon to attach itself and acquire a new stalk. Then the temporary girdle of cilia is 



Fig. 44. — Vorticella nebulif era, enlarged 

 about 600 times, a. Oilia. b. Cili- 

 ated disc. c. Peristome, d. Vesti- 

 bule, e. CEsophagus. /. Contractile 

 vacuole, g. Food vacuoles, h. Nu- 

 cleus. I. Endosarc. k. Ectosarc. 

 m. Muscle of stem. 



