Persons owning, keeping or having in their possession a dog or cat six months of age or older are required to obtain an annual license from the Humane Department. Such license shall be obtained between January 1 and April 1 of each year, at a cost of $10 per animal. If the license is not obtained until after April 1, the cost shall be $15. The additional fee provided for in this section shall not apply to dogs or cats brought into the city after April 1, provided a license for such dog or cat is obtained within 30 days after such dog's or cat's arrival in the city. This rule does not apply to dogs duly and properly trained to aid or assist physically disabled persons when any such dogs are actually being used by such persons for the purpose of aiding or assisting such person in going from place to place.