62 CEYLON PEARL OYSTER REPORT. 



seen indications, in the form of fine, slit-like canals in the closing membrane, of the 

 existence of several minute pores in the roof of each chone. 



From the primary inhalant canals the water reaches the flagellate chambers by fine, 

 sometimes branching canaliculi of considerable length (fig. 2, I.Ca.). The chambers 

 themselves (figs. 1, 2, F.C.} are sac-shaped, unbranched (at any rate usually) and up 

 to about 0'098 millim. long, though usually less (say about 0'065 millim.). They 

 open into the primary exhalant canals either directly (eurypylous) or through short 

 exhalant canaliculi. 



The mesoglcea, both of the ectosome and choanosome, is abundantly developed and 

 chiefly collenchymatous, with a profusion of small connective-tissue cells (fig. 2, C.T.C.) 

 with much-branched processes. The mesogloea of the ectosome passes insensibly into 

 that of the choanosome at the points of contact ; the former, however, stains rather 

 more darkly than the latter, and is often distinctly, though very finely, fibrillated. 



In the mesogloaa we also find embedded a large number of small, darkly staining, 

 spherical cells (figs. 1, 2, 3, Sy.A.), about 0'006 millim. in diameter, sometimes 

 apparently with small central nuclei. These cells are most abundant around the 

 various parts of the inhalant canal-system, and are congregated in immense numbers 

 in the inner portion of the ectosome. They sometimes appear to be broken up into 

 groups of smaller cells. They are in all probability symbiotic algae, such as are known 

 to occur frequently in sponges, a conclusion which is rendered almost certain by the 

 fact that, when treated with iodine and sulphuric acid, they give a distinct purple 

 coloration (although it must be admitted that when tested for starch with SCHULTZE'S 

 solution, only negative results were obtained). These bodies are probably closely 

 comparable to the "cellules spheruleuses " to which TOPSENT (loc. cit., p. 120) 

 attributes the red colour of his Hextidclla raeovitzai, and which are also (in my 

 opinion) probably symbiotic algae. Unfortunately, I have no information as to the 

 colour of Hexadclla indica in life, but if, as seems just possible, it is identical with 

 CARTER'S Halisarca rubitingens, it is also red, and the colour may likewise be 

 attributable to the symbiotic algae. 



This very interesting sponge appears to form a connecting link between the three 

 genera Osc-arellct, IIaUsarc<i, and Hexadella, to which latter genus 1 have referred it 

 only after long hesitation. It resembles the first in the distinctly folded character of 

 the choanosomal lamella and the arrangement of the flagellate chambers, which are, 

 however, much larger and usually (though not always) eurypylous, while a more 

 important distinction is introduced by the strong development of the ectosome. The 

 structure of the ectosome, the absence of the curious connective-tissue fibres, and the 

 unbranched character of the flagellate chambers, together with the less extensive 

 development of the mesogloea, separate it from Halisurca. The mesogkea appears, 

 however (as far as one can judge from TOPSENT'S description), to be a good deal more 

 extensively developed than in his species of Ilexaih'lla. TOPSENT says nothing about 

 the existence of u distinctly folded choanosome lamella, or of inhalant chones, in his 



