204 



CORALS 



traveller and used by him to distinguish a Lithophyllum 

 from a Lithothamnion. There is so much variation in size 

 and form in both genera, and colour is such an untrust- 

 worthy guide to generic distinctions, that there are many 

 specimens which can only be determined by experts in the 

 group. Nevertheless, there can be no doubt that when 

 critically examined the genera are distinct, and a few words 

 may now be written to indicate the nature of the characters 

 by which they are separated. When a thin section of a part 

 of a thallus of one of these genera is examined, it will be 

 found to consist of many layers of minute cells with thick 

 calcareous walls (Fig. 103). The cells are roughly cubical 



in shape and somewhere about 

 0-02 mm. in breadth. The layers 

 of cells are not uniformly ar- 

 ranged except in very young 

 growths, but exhibit oval or 

 spherical gaps that represent 

 the spaces in which the con- 

 ceptacles were placed. These 

 gaps may be about o-i mm. in 

 length. 



There are three kinds of con- 

 ceptacles, one kind containing 

 the tetraspores or asexual repro- 

 ductive bodies, a second kind 

 for the antheridia or male re- 

 productive organs, and a third for the female reproductive 

 bodies (archegonia or cystocarps). It seems probable that 

 no one specimen or frond of a specimen bears more than 

 one of these kinds of conceptacles at the same time. The 

 ripe sexual conceptacle in both genera is roughly dome- 

 shaped in vertical section, the dome usually indicated at 

 the surface by a convexity perforated in the centre by a 

 pore, and it is extremely difficult to distinguish the sexual 

 conceptacles of the one genus from those of the other by 

 any characters that persist in the dried coral. The young 

 tetrasporangial conceptacles, however, do show an important 

 difference. In Lithothamnion they are perforated by several 



Fig. 103. — Section of a piece of 

 the thallus of a Lithophyllum 

 showing the cells (about o-oa mm. 

 in breadth) with thick calcareous 

 walls and the surface (c, c) repre- 

 senting the gaps formed by old 

 conceptacles. 50 diams. 



