Introduction

The coelenterates have long been a favoured group for study by invertebrate marine zoologists. One reason for this is that these animals are abundant and usually easily obtainable. Another reason is that they are generally regarded as being the most primitive, in an evolutionary sense, of the metazoan animals, and it is believed that study of this group might reveal useful information regarding evolution of the lower metazoa. The coelenterates may be regarded (depending on one's opinion as to the correct evolutionary position of the group) as the first animals to show tissue specialization. Although sponges possess, for example, muscle cells (Hyman, 1940), these are not organised into muscle tissue or muscles such as one finds in coelenterates. Connective tissue in some coelenterates is also well developed (Chapman, 1953; 1966) and often has the appearance and properties of some vertebrate connective tissues. The coelenterates as a group thus often appeal to histologists as worthy of study, for many tissues seen in vertebrates make their first appearance here, and it could be expected that study of these tissues in coelenterates may lead to a better understanding of tissue structure and function in higher animals.

The present paper deals with structure and function of the column body wall of a sea anemone, Isactinia olivacea Hutton. It is one of a page 2 series of papers which study the body wall of various coelenterates and flatworms (phylum Platyhelminthes), and it is hoped that these studies may reveal information useful in a consideration of the phylogenetic relationship between these groups.

I. olivacea is found in the lower littoral zone of the sea shore attached to rocks, particularly in surge channels. An average sized specimen measures 1.5 cm. long with an oral disc diameter of 1 cm. The colour varies from olive green to brown.