STUDIES ON THE COMPAEATIVE ANATOMY OF SPONGES. 43 



the most complicated and irregular manner we find the pre- 

 cisely similar exhalant lacunae. The ultimate inhalant and 

 exhalant lacunae are separated only by narrow strands of gela- 

 tinous mesodermal tissue, in which lie the spicules and the fla- 

 gellated chambers (fig. 1). Thus every chamber lies wedged 

 in between an inhalant lacuna on the one side, and an exha- 

 lant lacuna on the other ; and it is only by noticing which 

 way the exhalant openings of the chambers are turned that it 

 is possible to tell whether a given lacuna is inhalant or exha- 

 lant. The exhalant lacunse gradually unite together into 

 larger and larger channels, and open finally on to the surface 

 of the sponge by means of the wide oscula, generally, if not 

 always, passing first into well-defined oscular tubes. 



The chambers themselves (fig. 1) may be roughly described 

 as subspherical. When, however, they are cut along the 

 imaginary axis running through the inhalant and exhalant 

 poles they frequently appear somewhat compressed, and we 

 see also that the exhalant opening (fig. 1, c, e. o. c.) is very 

 wide. When cut in a direction at right angles to this axis 

 they appear circular in outline, and the wall of the chamber is 

 uninterrupted (fig. 1, «). The diameter of the chamber in this 

 case is about 0'047 millimetre in length. (In the report of 

 the 'Challenger' Monaxonida the average diameter of the 

 chambers is given as 0*0336 millimetre. This figure appears 

 from my subsequent researches to be rather too low; but it 

 must be remembered that a certain amount of variation occurs 

 in this respect, in accordance with the state of preservation 

 of the specimens examined.) 



We now come to the most important consideration, viz. the 

 form and arrangement of the collared cells; and we will first 

 describe them as they appear when the chamber is seen in 

 section (fig. 1, a, c). The collared cells stand some little dis- 

 tance apart from one another on the gelatinous ground-sub- 

 stance surrounding the chamber. They have each a short 

 nucleated body, indistinguishable from the collum or neck, and 

 surmounted by the delicate funnel-shaped collar. The outlines 

 of the collars are extremely fine, but all the collars are con- 



