Question

what is tube within a tube plan​

Answer 1

It refers to the development of a fluid-filled cavity between the outer body wall and the digestive tube. The nature of this body cavity has led to the grouping of metazoa into three grades, acoelomate, pseudocoelomate, and eucoelomate (figure from Brusca and Brusca,1990) .

Answer 2

In this plan, the body has two tubes, one formed by the body wall and the second within it by digestive tract e.g., annelids.