VIII] OF THE SEPTA OF CORALS 623 



case an unsymmetrical one ; that is to say, there is one chamber which 

 is in contact with a greater number of its neighbours than any 

 other, and which at an earher stage must have had contact with 

 them all ; this was the case of our type i, in the eight-celled system 

 (p. 598). Such an asymmetrical chamber (which may occur in 

 a system of any number of cells greater than six) constitutes what 

 is known to students of the Coelenterata as a "fossula"; and we 

 may recognise it not only here, but also in Zaphrentis and its allies, 

 and in a good many other corals besides. Moreover, certain corals 

 are described as having more than one fossula: this appearance 

 being naturally produced under certain of the other asymmetrical 

 variations of normal space-partitioning. Where a single fossula 

 occurs, we are usually told that it is a symptom of " bilateraUty " ; 

 and this is in turn interpreted as an indication of a higher grade of 

 organisation than is implied in the purely "radial symmetry" of the 

 commoner types of coral. The mathematical aspect of the case 

 gives no warrant for this interpretation. 



Let us carefully notice (lest we run the risk of confusing two 

 distinct problems) that the space-partitioning of Heterophyllia by 

 no means agrees with the details of that which we have studied 

 in (for instance) the case of the developing disc of Erytkrotrichia: 

 the difference simply being that Heterophyllia illustrates the general 

 case of cell-partitioning as Plateau and Van Rees studied it, whil^ 

 in Erythrotrichia, and in our other embryological and histological 

 instances, we have found ourselves justified in making the additional 

 assumption that each new partition divided a cell into co-equal 

 parts. No such law holds in Heterophyllia, whose case is essentially 

 different from the others: inasmuch as the chambers whose parti- 

 tion we are discussing in the coral are mere empty spaces (empty 

 save for the mere access of sea-water); while in our histological 

 and embryological instances, we were speaking of the division of 

 a cellular unit of living protoplasm. Accordingly, among other 

 differences, the "transverse" or "periclinal" partitions, which were 

 bound to appear at regular intervals and in definite positions, when 

 co-equal bisection was a feature of the case, are comparatively few 

 and irregular in the earlier stages of Heterophyllia, though they 

 begin to appear in numbers after the main, more or less radial, 

 partitions have become numerous, and when accordingly these 



