SETOSELLA. 



691 ] 



SHELL. 



Char. Capsules quite sessile on a horny 

 polvpidom ; egg-germs contained in scattered 

 deciduous capsules. British genera : 



Halecium. Polypidom plant-like ; stem 

 consisting of several parallel tubes ; cells 

 shallow cups, on opposite sides, alternate, 

 one under each joint. 



Sertularia. Plant-like ; stem simply tu- 

 bular, branched, jointed; cells vase-like, 

 alternate or in pairs, on opposite sides, with 

 everted rims. 



Reticularia. Polypidom an investing net- 

 work of horny tubes, immersed in a homo- 

 geneous horny crust; cells short curved 

 projections of the tubes, with simple round 

 orifices. 



Coppinia. Parasitic, massive, hairy ; cells 

 long, tubular, often curved, arising at irre- 

 gular distances (generally at the angles of 

 junction) out of a cellular basis, the aper- 

 tures of the cells or spores of which are often 

 themselves covered in by a lid perforated 

 by a small tubular orifice. 



Thuiaria. As Sertularia, but the cells 

 closely pressed to or imbedded in the surface 

 of the" stem and branches. 



Antennularia. Simple or branched, j ointed, 

 with slender hair-like whorled branchlets ; 

 cells small cups on the inner side of the 

 branchlets ; egg-vesicles seated in the angles. 



Plumularia. Simple or branched, feathery ; 

 cells small, usually in the angles formed by 

 horny spines on the inner side of the branches; 

 egg-vesicles scattered. 



BIBL. Hincks, Hydr. Zooph. 233. 



SETOSEL'LA, Hks. A genus of Poly- 

 zoo= Membranipora pt. S. vulnerata, on 

 very small stones in deep water. (Hincks, 

 Polyz. 181.) 



SHEEP-TICK. See MELOPHTLA. 



A species of Trichodectes (sphcerocephalus) 

 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. See PI. 45. figs. 1-16. 



Egg-shell. As an example of the struc- 

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

 the shell of the egg of the common fowl. 



This is lined internally by a loosely ad- 

 herent 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 

 imperfectly formed or soft eggs, as they are 

 called, the fibres present thickenings at irre- 

 gular intervals, resembling, on the whole, 

 the nuclear 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, in- 

 separably 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 ad- 

 herent masses (PI. 45. 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. 45. fig. 13) ; this 

 is most easily perceived in the inner por- 

 tions of the shell. 



The structure of the shell of the ostrich 

 presents a curious variety. In a section 

 parallel to the surface (PI. 45. fig. 14) the 

 protoplast structure is visible, but the cal- 

 careous matter is arranged in the form of 

 triangular plates, often fused together, and 

 leaving angular interspaces. The perpen- 

 dicular section is represented in PL 45. 

 fig. 15. The former section constitutes an 

 interesting polarizing object. 



Tortoise-shell. This substance is an epi- 

 dermic formation, structurally resembling 

 horn, in so far 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 necessary 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 

 respective varieties has been used as an aid 

 to the recognition of fossils, and the de- 

 termination 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 polv- 

 gonal prisms, placed sometimes perpendi- 

 cularly, sometimes obliquely to the surface 

 2Y 2 



