SHEEP-TICK. 



[ 575 ] 



SHELL. 



tube, denticulated or serrated with the cells, 

 and jointed at regular intervals ; cells alter- 

 nate, semi-alternate or opposite, biserial, 

 sessile, urceolate, short, with everted aper- 

 tures; ovarian vesicles scattered; polypes 

 hydriform. 



Many of these elegant zoophytes, which 

 would at once be referred to the vegetable 

 kingdom by any casual observer, are com- 

 monly found on the sea-coast, either loose 

 or attached to shells, sea- weeds, &c. Only 

 on examination with a lens or microscope 

 can the cells in which the polypes were 

 contained be found ; and the polypes them- 

 selves are rarely to be met with. 



There are seventeen British species (John- 

 ston). 



5. rugosa (PL 33. figs. 11 & 12). Cells 

 alternate, one to each joint, ovate, trans- 

 versely wrinkled, mouth narrow, with four 

 small'teeth at the rim. 



Common upon Flustra, Fuci, &c. at low- 

 water mark. 



S. pumila (PI. 33. figs. 13 & 14). Cells 

 opposite, approximate, shortly tubular, the 

 top everted, with an oblique somewhat mu- 

 cronate aperture ; vesicles ovate. 



Common on Fuci near low-water mark. 



S. operculata (PI. 33. figs. 1 5 & 16). Cells 

 opposite, inversely conical; aperture patu- 

 lous, obliquely truncate, pointed near the 

 outer edge, and with two small lateral teeth ; 

 vesicles obovate. 



Common on Fuci near low-water mark. 



BIBL. Johnston, Brit. Zoophytes, 61. 



SHEEP-TICK. See MELOPHILA. 



A species of Trichodectes (spharocephalus) 

 is also found as a louse upon sheep. 



SHELL, OF ANIMALS. In this article 

 we shall notice the various substances com- 

 prised under the term shell, in its common 

 acceptation. 



Egg-shell. As an example of the struc- 

 ture of the egg-shell of birds, we may se- 

 lect the shell of the egg of the common 

 fowl. 



This is lined internally by a loosely adhe- 

 rent layer of a thin yet firm albuminous 

 membrane, called the membrana putaminis. 

 It consists of a number of very slender fibres, 

 interlacing in various directions. In imper- 

 fectly formed or soft eggs, as they are called, 

 the fibres present thickenings at irregular 

 intervals, resembling, on the whole, the nu- 

 clear fibres of elastic tissue with the remains 

 of their formative cells still visible. On 

 macerating the shell in dilute muriatic acid, 

 an outer layer of this membrane, inseparably 



adherent in the natural state to the inner 

 surface of the shell, may be detached. 



The membrane may be heated to boiling 

 in solution of potash, without undergoing 

 solution, and is insoluble in acetic acid ; but 

 it is coloured by Schultze's test. 



The substance of the shell consists of nu- 

 merous 'masses of secretion or protoplasts, 

 impregnated with calcareous matter. In 

 soft eggs, these form rounded, loosely adhe- 

 rent masses (PI. 37. fig. 12), may easily be 

 detached from the surface of the egg, and 

 contain but little calcareous matter ; whilst 

 in the perfect egg they are somewhat angu- 

 lar from mutual pressure, and abound in 

 calcareous granules having an imperfectly 

 radiating arrangement (PI. 37. fig. 13) ; this 

 is most easily perceived in the inner portions 

 of the shell. 



The structure of the shell of the ostrich 

 presents a curious variety. In a section 

 parallel to the surface (PI. 37. fig. 14) the 

 protoplast structure is distinctly visible (al- 

 though omitted in the figure), but the cal- 

 careous matter is arranged in the form of 

 triangular plates, often fused together, and 

 leaving angular interspaces. The perpendi- 

 cular section is represented in PI. 3/. fig. 15. 

 The former section constitutes an interesting 



Tortoise-shell. This substance is an epi- 

 dermic formation, structurally resembling 

 horn, insofar as it consists of epidermic 

 cells, flattened, and united into numerous 

 superimposed plates. The long-continued 

 action of solution of potash (from twenty- 

 four to forty-eight hours), and the subse- 

 quent addition of water, are necessaiy to 

 resolve tortoise-shell into its component 

 cells. 



Shells of the Mollusca. The structure of 

 these shells varies in the different orders, &c. 

 of the class ; and a knowledge of the respect- 

 ive varieties has been used as an aid to the 

 recognition of fossils, and the determination 

 of the affinities of the genera, families, &c. 



In the bivalve Mollusca, two kinds of 

 structure may be distinguished, an outer 

 prismatic or fibrous, and an inner lami- 

 nated. 



The outer prismatic portion consists of 

 flattened masses or plates of crowded poly- 

 gonal prisms, placed sometimes perpendicu- 

 larly, sometimes obliquely to the surface of 

 the inner layer. These prisms are transpa- 

 rent and polarize light, possessing a crystal- 

 line structure, although their forms are not 

 crystalline, but those resulting from mutual 



