148 TABULATE CORALS. 



that the tube can be the work of some parasitic animal which 

 bored into the coral after the latter was formed. 



3. The tube was by no means confined to the mere base of 

 the coral, nor did it lie like the letter S in a single plane. On 

 the contrary, it traversed the corallum in a loose and open 

 spiral, so that both vertical and horizontal sections cut it at 

 various points (see PL VIII., figs, i-i b). 



4. The tube (in adult specimens, at any rate) was completely 

 concealed within the corallum, and was, to all appearance, alto- 

 gether superior to the epitheca. Whether or not it had any 

 opening upon the upper surface of the corallum, I cannot say. 1 



5. The walls .of the tube are extremely thin, calcareous, and 

 apparently destitute of any definite structure. The interior is 

 usually filled with transparent calcite, but occasionally there are 

 seen inside it numerous spheroidal or ovoid opaque bodies, as 

 to the nature of which I can offer no opinion whatever (PI. 

 VIII., fig. i a). It will be seen from the above that this 

 peculiar structure is one well worthy of further examination 

 by those who may have calcareous specimens suitable for the 

 preparation of microscopic sections. My own observations are 

 much too limited to enable me to come to any definite conclu- 

 sions as to its true nature ; but they are at the same time suffi- 

 cient to make me dubious as to the received explanations upon 

 this subject. 



Returning now to P. stylophorum, Eaton, I may give a brief 

 account of its structure, as shown by microscopic sections, it 

 being the only species of Pleurodictyum which, to my know- 

 ledge, has ever been examined by this method. The general 

 form of the corallum, as seen in fig. 22, c and D, is that of a 

 small discoid expansion, the upper surface of which carries the 

 calices, and is more or less convex ; while the lower surface is 

 generally flat or slightly convex, and is covered by a strong 



1 Dr Rominger (Amer. Journ. Sci. and Arts, vol. xxxv. p. 82), however, states 

 that he has observed specimens in which the vermiform tube opens upon the upper 

 surface of the corallum by a round mouth. His further statement that the tube 

 traverses the substance of the corallum irrespective of the direction of the corallites 

 through which it seems to cut, is not borne out by my specimens. 



