The Microscope. 



109 



dom is traced is anatomically much like a sea-anemone or other 

 actinozoan. It is sac-like in shape, and has, on one of the sur- 

 faces, an elongate opening,which serves not only as mouth, but 

 for the rejection of the faeces as well. Surrounding this mouth 

 is a ring of epiblast, specialized as a nervous system. Cutting 

 a section below the mouth (in the position the animal usually 

 occupies), we find a central digestive cavity, and radiating from 

 it a series of pouches (p) by which the assimilative surface is 



Fig. 2. Diagram of the structure of a sea-anemone, the section passing 

 through the radial partitions and the digestive cavity, c, cinclides or openings 

 from the archenteric pouches (p) 10 the exterior; I, longitudinal canals passing 

 from one pouch to another; m, mouth; n, nervous ring; the last two being in a 

 plane above the rest of the section. This represents Sedgwick's ancestral form. 



greatly increased. These pouches are connected with one an- 

 other by openings through the partitions (£), and each (in some 

 genera) with the exterior by other apertures, the so-called cin- 

 clides (c). These points are all of the anatomical features that 

 need here be noticed. 



There are some embryological points which need mention. 

 The actinian is but little removed from the gastrula ; its 

 " mouth " is the blastopore, and its digestive cavity is the arch- 

 enteron. To gain additional absorptive surface, the archenteric 

 walls become folded so that they form a series of pouches con- 

 nected with the central cavity ; and it is to be noted that these 

 pouches, in their mode of origin, are comparable to the ccelo- 

 matic pouches of Amphioxus, previously described. They are 

 diverticula of the primitive stomach of the gastrula. 



