. The Cambridge natural history. Zoology. 208 PORIFERA ends of the sixth rays, and at present problematic. An axial canal is present in each of the raysâthe six canals meeting at the centre of the spicule. Special chinks between the spicules appear to have provided a passage for the water current. The beautiful Ventriculites, so common in the Chalk and present in the Cambridge Greensand, are historically interesting, for the fact that they are fossil Hexactinellida of which the general and skeletal characters were very minutely described by Toulmin Smith long before recent representatives of t
. The Cambridge natural history. Zoology. 208 PORIFERA ends of the sixth rays, and at present problematic. An axial canal is present in each of the raysâthe six canals meeting at the centre of the spicule. Special chinks between the spicules appear to have provided a passage for the water current. The beautiful Ventriculites, so common in the Chalk and present in the Cambridge Greensand, are historically interesting, for the fact that they are fossil Hexactinellida of which the general and skeletal characters were very minutely described by Toulmin Smith long before recent representatives of the group were known. In common with a num- ber of fossil Dictyonine species they are distinguished by the per- foration of the nodes, a character due to the fact that the siliceous investment which unites the spicules together stops short before reaching the centre of each spicule, and bridges across the rays so as to form a skeleton octahedron. This character is rare in recent Fig. 103.âa node ot the skeleton of Hexactinellids, but, as first pointed Ventriculites from the Cambridge o^^ ^y Carter, it is presented by Greensaud. (After SoUas.) â ' ' f. , . i y , one or two forms, of which Aulo- cystis grayi Bwk is best known. The majority of the fossil Hexactinellida belong to the Dictyonine section, a fact attribut- able to the greater coherence of their skeleton. The " Dictyonina " are to be reckoned among the rock-builders of Jurassic and Cretaceous times. The Octactinellida and Heteractinellida are two classes created by Hinde^ to contain certain little-known Devonian and Carboniferous sponges, possessing in the one case 8-rayed spicules, of which 6 rays lie in one plane and 2 are perpendicular to this plane; in the othfer case, spicules with a number of rays varying from 6 to 30. Bearing in mind the manner in which octactine spicules are known to arise in recent Hexactinellida (p. 200), it is clearly possible to derive these 8-rayed spicules from hexactines by s
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
