ON A FREE-SWIMMING HYDROID. 11 



worthy that two of these large nuclei may be found lying 

 close together, side by side, on the same side of one septum, 

 which seems to indicate that each cavity in the axial tissue is 

 not simply the enlarged vacuole of a single cell. Though 

 abundant in the tentacles themselves, the large nuclei are, so 

 far as my experience goes, not to be found in the vacuolated 

 mesogloea with which the axial tissue of the tentacle becomes 

 continuous in the proboscis wall. 



Owing partly to the specimen being somewhat injured in 

 the neighbourhood of the mouth (possibly by being washed 

 about by the tide on the sand, with mouth downwards), I 

 have been unable to make a satisfactory investigation of the 

 minute structure of the smallest tentacles. It is evident, 

 however, that these conform much more closely to the ordi- 

 nary Tubularian type than do the large ones. This may be 

 chiefly owing to their smaller diameter, which enables the 

 membranous septa to stretch right across transversely and 

 more or less parallel with one another, so as to divide the 

 interior into approximately a single row of chambers, sur- 

 rounded by a very thick layer of mesogloea inside the 

 ectoderm. Thus it would seem that the axis of the smallest 

 tentacles is occupied by a single row of large vacuolated 

 endoderm cells as usual. Whether even in the smallest 

 tentacles these axial cells retain their connection with the 

 endodermal lining of the gastral cavity is extremely doubtful. 

 In the case of the large tentacles there is no trace of any 

 connection remaining between the axial tissue and the endo- 

 derm of the gastral cavity,^ and the origin of this tissue 

 must remain doubtful. It has probably been originally 

 derived from the endoderm, but it has become so modified in 

 structure and so completely disconnected that perhaps only 

 embryological research can decide the question. 



Wall of the Float. — The wall of the float forms but a 

 comparatively thin shell, enclosing the central cavity with its 

 remarkable system of supporting membranes. The histo- 

 logical characters of the ectoderm (fig. 12, Ect.) are very 

 * Coiupdic Mi^ajima's remarks on Braucliioceriautlius, loc. cit. 



