. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 566 SHELL. which connect these plates. The Sep'wstaire havin-; been formerly described in some detaif (vol. i., p. 54-6), it will only be re- quisite here to mention, that the calcified layers which alternate with horny membranes to form the shallow cone or cup, exhibit a distinct cellular structure, when the section is made sufficiently thin ; and that indications of a similar structure may also be perceived in the delicate and fragile plates which are arranged obliquely upon one another in the hollow of this cup. Few o
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 566 SHELL. which connect these plates. The Sep'wstaire havin-; been formerly described in some detaif (vol. i., p. 54-6), it will only be re- quisite here to mention, that the calcified layers which alternate with horny membranes to form the shallow cone or cup, exhibit a distinct cellular structure, when the section is made sufficiently thin ; and that indications of a similar structure may also be perceived in the delicate and fragile plates which are arranged obliquely upon one another in the hollow of this cup. Few of the numerous fossil shells referable to this class have yet been examined ; it may, however, be stated as an interesting result of microscopic ob- servation, that the " spathose guard" of the Belemnite is thereby proved to be composed of long prismatic cells, radiating from the centre to the circumference; closely re- sembling in their general arrangement those of the massive tube of Septaria g/gnntea, the great sand-boring Teredo of Sumatra. The structure of the shells of the testa- ceous Annelida, and of the pedunculate Cirrlw- poda, does not essentially differ from that of Mollusca; but in most of the sessile Cir- rhopods, such as the common Balanus, we find a cancellated structure or diploe in- tervening between the inner and outer plates of the shell (vol. i., p. 685). A less regular Fig. 421. verse partitions (fig. 422.). The cancelli are frequently occupied by calcareous infiltra- Fig. Cancellated structure from shell of Hippurite, as seen in transverse section. Magnified 5 diameters. diploe has been described by Mr. J. E. Gray* as existing between the laminae of Ostren piirpurca; but in no other shells of existing Mollusca has any approach to it been yet discovered. A very regular cancellated structure, however, is exhibited in the singular extinct group of Rud'mtcs, where it makes up nearly the entire thickness of the shell (fig. 421.). The cancelli
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