340 DESIGN IN NATURE 



(/) A large number of food globules and food particles. These are seen to advantage when the animal is 

 artificially fed on carmine, as was done in the case represented by Fig. 74. 



All the parts enumerated are of the simplest description. The animal is unicellular and composed of proto- 

 plasm, no structure in the ordinary sense being anywhere perceptible. Notwithstanding its extreme simplicity it 

 can live, secure and assimilate food, grow, and reproduce itself. The manner in which it suppUes itself with food 

 is the most ingenious and effective possible. It causes its flagellum to perform a rotatory vibratile movement within 

 the funnel-shaped collar in such a manner as to produce in the water in which it lives a series of well-directed 

 currents, which cause the food particles in its vicinity to travel upwards on the outside of its collar and then to turn 

 over and travel downwards on the inside of the collar until they reach the body, where they are ultimately incor- 

 porated. Here we have an example of a simple organism performing voluntary co-ordinated movements to a given 

 end, no trace of nerve, muscle, gland, or other structure being present. The movements of the flagellum are 

 evidentiy controlled, and they are the outcome of arrangement and design as surely as are the movements of the 

 higher animals devised for a similar purpose. Nor are the means of digesting the food so scientifically secured less 

 effective. There is no stomach, nevertheless the food is taken into the protoplasm of the body and digested and 

 assimilated, and -the effete or waste products extruded without difficulty. As a matter of fact, every part of the 

 body becomes a stomach fro tern, when food has to be negotiated. 



That the monad is a hving entity is proved by its life history and by the presence of a nucleus and contractile 

 vesicles in its body ; these being entrusted with important functions. The latter (contractile vesicles) pulsate 

 rhythmically, and are endowed with centrifugal and centripetal movements like the heart in the higher animals. 

 The body and collar are endowed with similar movements. All parts of the monad move independently. The nucleus 

 during the reproductive process divides into two ; it, with the protoplasm of the body, ultimately becoming two 

 separate individuals. Eeproduction in the monad may occur in three ways : (a) by longitudinal division, (6) by 

 transverse division, and (c) by the animal assuming the amoeboid and encysted forms prior to the cleavage. It 

 occasionally happens that two monads unite and fuse and become one individual. All the functions of hfe are 

 satisfactorily discharged by the monad, simple though it be. 



§65. Vorticella. 



The vorticella is quite as interesting and instructive as the monad. This is one of the most elegant, active, and 

 wonderful of rudimentary animals. It can just be seen with the naked eye, and occurs both in salt and fresh water. 

 The salt and fresh-water specimens closely resemble each other. The following account appUes mainly to the larger 

 fresh water-forms. 



A fresh-water vorticella in the expanded condition consists of : — 



(a) A cup-shaped body with a spiral stem, by which it fixes itself to plants or other objects, and by which it 

 can advance or withdraw its body at pleasure. Occasionally it dispenses with the stem and swims freely. 



(&) A row of ciha attached near a prominent spiral rim (peristome) at the upper, distal, or free end of the body. 



(c) A vestibule and pharynx conducting to the ventral surface of the body. 



{d) An anus through which faecal matters are extruded at intervals. 



(e) Food-vacuoles which circulate within the endosare or internal fluid protoplasm. 



(/) A pulsating vacuole endowed with centripetal and centrifugal movements. 



(g) An elongated, curved, rod-like nucleus (Fig. 75). 



The vorticella is a protozoon, and the entire animal (the stem included) consists of a single cell, in which respect 

 it resembles the amoeba, gromia, and paramecium already described. The body is composed of protoplasm or 

 sarcode, which as in paramecium is divided into an outer, firmer layer (ectosarc) and a central, more fluid mass (endosare). 

 The stalk, which is supplied with a spiral, contractile fibre or band, is composed of ectosarc alone. The stalk of 

 the vorticella affords a beautiful example of a specially-constructed spiral spring, the movements of which are 

 regulated by the animal. They are very little due to elasticity. 



The ciha, which are situated in a spiral groove between the disc and peristome, and also in the vestibule and 

 pharynx, are exceedingly sensitive, and can be moved voluntarily in consecutive order and in specific directions ; 

 the object being to produce fluid currents which sweep food into the pharynx and body. The cihary movements 

 are co-ordinated and spontaneous in a marked degree, and the same is to be said of the spiral movements of the 

 stem. These are perfectly under control, and no more beautiful sight can be witnessed than is afforded by a group 

 of vorticellse feeding under natural conditions. The ciha are in constant motion, and the body is advanced gently 

 and made to grope about in search of food. If it touches anything suspicious or doubtful it is instantly withdrawn, 

 to be again cautiously and slowly projected. As to the voluntary nature of all the movements of the vorticella there 



