556 



COLLEGE ZOOLOGY 



but later they become attached to some sohd 

 object for Hfe. Certain adult sponges are 

 capable of slight contractions of the body 

 and of feeble bending movements; but 

 greater activity occurs within the body, due 

 to contractile cells, which are arranged about 

 the osculum and pores. Stimuli of various 

 sorts applied directly to the contractile cells 

 result in the opening and closing of the 

 osculum, pores, and canals, and in changes 

 in the flow of water. Transmission to other 

 oscula does not occur. There appears to be 

 present, however, a sluggish type of trans- 

 mission through the protoplasm, such as 

 seems to occur in certain protozoans. This 

 is transmission, without highly differentia- 

 ated nervous elements, such as are present 

 in the usual nerve transmissions. Studies of 

 the behavior of sponges indicate that no true 

 neurons exist. Sponges, therefore, have cells 

 that act as effectors but do not possess spec- 

 ial receptors or adjustors. 



branches, and the motor neurons may come 

 in contact with several sensory neurons by 

 means of their dendrites. Such contacts are 

 known as synapses. Besides this, association 

 neurons are present in the central nerve cord, 

 and nerve impulses may pass from one of 

 these to another throughout the entire 

 length of the cord. This association of neu- 

 rons results in complex reflex arcs which are 

 characteristic of all the higher animals. The 

 cerebral ganglia at the anterior end of the 

 earthworm are slightly larger than the gan- 

 glia elsewhere as a result of the concentra- 

 tion of sensory neurons in that region of the 

 body. It is the anterior end which first en- 

 counters changes in the environment during 

 locomotion. 



The concentration of nerve cells in the 

 head region, which is known as cephaliza- 

 tion, continues as one proceeds up the 

 animal series, culminating in the large brain 

 of man. 



Hydra 



A distinct advance in behavior is evident 

 in the hydra, where specialized nerve cells 

 form a sort of network in the body wall, 

 which enables the animal to perform com- 

 plex coordinated movements of the entire 

 body. Simple reflexes in hydras are those 

 that involve movement of a part of the 

 body that is stimulated, for example, bend- 

 ing a single tentacle. Reception of stimuli 

 and transmission of nerve impulses by sen- 

 sory cells, conduction, and discharge of im- 

 pulses by neurons, with the resultant action 

 of effectors such as contractile and secretory 

 cells, all occur in the hydra. 



Earthworm 



In the earthworm we encounter a highly 

 developed nervous system and definite sense 

 organs. Most of the nerve cells are concen- 

 trated in the ganglia of the nerve cord. Here 

 a sensory neuron may come in contact with 

 several motor neurons by means of axon 



Instinct 



The reactions of the earthworm to vari- 

 ous external stimuli are much more complex 

 than those of the hydra. As in all animals, 

 they are for the most part of benefit to the 

 animal, and vary according to the physio- 

 logic state of the individual. Such reactions 

 as withdrawal of a worm into its burrow, as 

 a result of vibration or of sudden illumina- 

 tion at night, are due to compound reflexes 

 and are commonly spoken of as instincts 

 These instincts consist of a chain of co- 

 ordinated reflexes of the organism. Instinc- 

 tive acts are performed without any pre- 

 vious experience and are characteristic for 

 each type of animal. For example, certain 

 caterpillars spin cocoons, spiders spin webs, 

 birds build nests, and honey bees construct 

 honeycombs. In many cases such instinctive 

 acts are carried out only once during the 

 lifetime of the organism. This type of be- 

 havior is dependent on inherent reflex arcs, 

 and instincts are often referred to as in- 

 herited behavior; each species of animal 



