76 



COMPARA TIVE AX A TOMY 



CHAP. 



to fuse with its walls, so that the gastric cavity here appears divided 

 into chambers or compartments arranged radially round the cesophageal 

 tube (Fig. 66, to the left). 



In the remainder of the body (Fig. 66, to the right) the septa pro- 

 ject their free inner edges into the gasti'ic cavity, which thus appears 

 divided into a central portion and into radial niches separated by the 

 septa (analogous to the gastro-canals of the other Cnidaria). These 

 niches, in the neighbourhood of the cesophageal tube, are continued 

 direct into the radial compartments between the tube and the body 



FIG. 06. Diagrammatic transverse section of a Coral individual ; to the left on the level of 

 oesophagus, to the right on the level of the gastric cavity. -&, Direction of the plane of sym- 

 metry. 



wall, and these compartments again are continued into the axial cavities 

 of the tentacles, which are lined with endoderm (cf. Fig. 82, p. 107). 

 The axial cavities of the tentacles occasionally open externally at the 

 tip by a pore. 



The number of tentacles represents, generally, the number of the 

 septa. The Octocorallia have 8 septa, and 8 tentacles placed so as 

 to alternate with them. The Tct racondlm generally have a large 

 number of septa which are always a multiple of 4. The HexacoraUia 

 possess 6 or 611 partition walls and tentacles, arranged in a definite 

 order which cannot here be described. We can only say, quite gener- 

 ally, that the oldest septa project farthest, and the youngest septa least 

 far, towards the axis. 



Most Corals have not a strictly radial structure ; on the contrary 

 we often find, anatomically and ontogenetically, a bilateral symmetry 

 in the arrangement of the parts of the body. The slit-like shape of 

 the mouth even is a departure from radial structure. A plane through 

 the chief axis of the body in the longitudinal direction of the mouth 

 (Fig. 66, a-b) is, in fact, a median plane the only plane which divides 

 the body into two exactly similar halves. 



More exact ontogenetic investigation has shown that (e.g. in the 

 Hexacomllia) two septa lying opposite each other to the right and left 

 of this plane are first formed ; these septa incompletely divide the 

 gastric cavity into two portions of unequal size. Two new septa are 

 then formed symmetrically in the larger say the anterior division ; 



