IDIOSTROMA OCULATUM. 227 



transverse sections of the stems ; but in longitudinal sections they are usually 

 badly shown, apparently owing to their tortuous character. 



Many specimens of Idiostroma oculatum, exhibit embedded tubes which have 

 the general character of " Caunopora-tubes," since they open on the surface by 



Fig. 33. 



Fig. 33. — Transverse section of half of a stem of Idiostroma oculatum, Nich., 

 enlarged about 12 times, a. The axial canal transversely divided, b b. Large 

 radially directed tuhes embedded in the ccenosteum, opening inferiorly into the 

 interlaminar spaces, but acquiring thickened walls, and becoming intersected by 

 tabula where they approach the surface. 



rounded apertures with thickened margins, and are provided with proper walls 

 (woodcuts, figs. 32 a and 33). Other examples show no traces of these embedded 

 tubes (Plate XXIX, fig. 8). Sections of the stems (Plate XXIX, fig. 11, and 

 woodcut, fig. 33) show that these supposed " Caunopora-tubes " possess a 

 thickened proper wall in the outer part of their course, but apparently lose this 

 as they are traced internally, till they appear in some cases to open directly into 

 the interlaminar spaces of the coenosteum. Doubt is thus thrown upon the true 

 nature of these embedded tubes ; and if they were constantly present there would 

 be good ground for thinking that they really belong to the organism with which 

 they are associated. This conclusion is, however, rendered doubtful by the fact 

 that some specimens do not show any traces of the presence of these tubes ; and, 

 in spite of a laborious investigation, I must still confess myself as unable to come 

 to any positive conclusion as to whether or not these embedded tubes are adven- 

 titious. In view, however, of the extent to which the calcareous tissue of a shell 

 enclosed in a crust of the recent Hydractinia echinata may be eaten away or 

 absorbed by the parasite, I feel inclined to doubt if any stress can be laid upon 

 the apparent absence of a proper wall to the embedded tubes of Idiostroma 

 oculatum in the more deeply buried part of their course. 



Obs. — The only other species of the genus Idiostroma with which I have any 



31 



