Currently, only one species, Trichoplax adhaerens, is acknowledged to belong to the Placozoa. The Italian Francesco Saverio Monticelli described another species in 1893, which he found in the waters around Naples, naming it Treptoplax reptans.
A standard mallet has a head length of 9 to 9.5, and this is recommended for beginners. Expert players may use longer head lengths up to 12, as this is thought to make aiming more accurate and makes it harder to accidentally twist the mallet during the stroke.
There are different sizes of tennis rackets starting at 19 for a young player and go up to 27 which is a full size racket. In my opinion Slazenger is a particular good brand of mini tennis racket, however your Tennis Coach will be able to advise you on a suitable brand.