. The Cambridge natural history. Zoology; Zoologia Geral. 20O PORIFERA the same layer, added to the more obvious character of thimble- shaped chambers, are the chief archaic features of Hexactinellid morphology. The skeleton which supports the soft parts is, like them, simple and constant in its main features. It is secreted by scleroblasts, which lie in the trabeculae, and is made up of only one kind of spicule and its modifications. This is the hexac- tine, a spicule which possesses six rays disposed along three rect- angular axes. Each ray contains an axial thread, which meets its fe
. The Cambridge natural history. Zoology; Zoologia Geral. 20O PORIFERA the same layer, added to the more obvious character of thimble- shaped chambers, are the chief archaic features of Hexactinellid morphology. The skeleton which supports the soft parts is, like them, simple and constant in its main features. It is secreted by scleroblasts, which lie in the trabeculae, and is made up of only one kind of spicule and its modifications. This is the hexac- tine, a spicule which possesses six rays disposed along three rect- angular axes. Each ray contains an axial thread, which meets its fellow at the centre of the spicule, where they together form the axial cross. Modifications of the hexactine arise either by reduc- FIG. 91. — Portion of a section of the . , membrana reticularis or chamber- tlOll or branching, by SpinulatlOn or expansion of one or more of (After F. E. Schulze.) . the rays. The forms of spicule arising by reduction are termed pentactines, tetractines, and so on, according to the number of the remaining rays. Those rays which are suppressed leave the proximal portion of their axial thread as a remnant marking their former position (Fig. 94). Octactine spicules seem to form an exception to the above state- ments, but Schulze has shown that they too are but modifications of the hexactine arising by (1) branching of the rays of a FIG. 92.—A, discohexaster, iu which the four cladi «, «', b, b', c of each ray start directly from a cen- tral nodule. B, disco- octaster, resulting from the redistribution of the twenty-four cladi of A into eight groups of three. (After Schulze. from Delage.). Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Harmer, S. F. (Sidney Frederic), Sir, 1862-; Shipley, A. E. (Arthur Everett), Sir, 1861-1927. [London, Macmil
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