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THE BRITISH NATURALIST [o 



animal. Instead of any part being capable of serving as a temporary 

 mouth we have here a definite mouth. This mouth has become a 

 necessity, for the ectosarc of Vorticella is, compared with that 

 of Amoeba, rigid. The bell of Vorticella is to a certain extent 

 contractile, but there is a specially contractile element found in 

 the stalk. This stalk is. a hollow cylindei, on the inner wall of 

 which, coiled spirally around it, is a thickened ridge of protoplasm. 

 Though the Avail of the stalk itself is not contractile, this ridge of 

 protoplasm is highly so, and its contraction has the effect of throwing 

 the stalk into a spiral, and drawing the bell down to its attachment. 

 If any foreign body, too large to be taken into the mouth, or otherwise 

 objectionable, comes in contact with the cilia around the mouth, w r e see 

 the animal suddenly retracted by the contraction of this layer of 

 protoplasm in the stalk. Here we have the rudiments of a tactile j 

 sense organ sending impulses to definite muscles such as we are 

 familiar with in the higher animals. We cannot trace the course of the j 

 nerve current from the disc to the stalk, but we may assume that it 

 runs along one track more readily than another, and that if this track 

 were still more specialized we should recognize it as a rudimentary 

 nerve. The presence of a definite mouth in Vorticella suggests that the 

 power of distinguishing food from inorganic matter is specialized j 

 around this mouth ; in other words that Vorticella has a somewhat 

 specialized organ of taste. 



Let us now examine a fairly simple multicellular animal. Hydra 

 presents no special sense organ, unless the cells of the outer 

 layer of the body and tentacles are to be regarded as tactile organs, j 

 A section of Hydra shows the body wall to consist of three layers. 

 — an external layer, the ectoderm ; a middle muscular 'layer, the 

 mesoderm ; and an inner digestive layer, the endoderm. 



The cells found in the ectoderm give strength to the body wall, and 

 being on the outside are exposed to changes in the environment ; the 

 deeper parts of these cells end in tail-like processes which run in the 

 mesoderm, and their presence in the mesoderm gives to this layer its 

 character as the muscular layer. The relation of the contractile part 

 of these ectoderm cells to the sensitive part is thus closely analogous i 

 to the relation of the stalk to the body of Vorticella. In Hydra there 

 are no definite nerves ; the current of molecular disturbance must pass 

 from the sensitive part to the motor part in the substance of the cell , 

 itself. A condition has been described in some of the Hydrozoa in which 

 the connection between the sensitive and contractile part of the ectoderm 

 cell is drawn out into a fine thread, which will; in fact, represent a 

 primitive nerve. We only require the development of a nerve cell in 

 the course of this nerve to reach the typical condition of sense 

 organ, nerve, nerve cell, and muscle already referred to. These 



