Leathesia difformis, the sea potato, is a littoral brown alga in the class

Class (biology)

In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 Phaeophyceae and the order Ectocarpales

Ectocarpales

Ectocarpales is a very large order in the brown algae . The order includes families with pseudoparenchymatous or true parenchymatous tissue...

 which is commonly attached to other seaweeds and sometimes rocks. When young, the organism is solid but as it matures it become hollow and somewhat convoluted and has the appearance of a small leathery brown bag about the same size as a tennis ball

Tennis ball

A tennis ball is a ball designed for the sport of tennis,approximately 6.7 cm in diameter. Tennis balls are generally bright green, but in recreational play can be virtually any color. Tennis balls are covered in a fibrous fluffy felt which modifies their aerodynamic properties...

. The texture is rubbery and the outer surface smooth.