What is responsible for the differences between Pelmatohydra

What is responsible for the differences between Pelmatohydra & Chlorohydra?

Gastrodermal zoochlorellae are responsible for green color in Chlorohydra and these are absent in Pelmatohydra. Pelamatohydra is brown in color and it is different from normal hydra by division of its column into a narrow stalk at proximal end and thick distal body.