HA RD WI CKE 'S S CI EN CE - G OS SI P. 



265 



THE HARD PARTS OF ANIMALS-No. H. 



By Dr. IT. F. PARSONS, F.L.S. 



N the Subkingdom Mol- 

 luscoida, the Bryozoa in 

 external form closely 

 resemble the Polypes, 

 from \\'hich they have 

 only of late years been 

 separated. They have 

 a common horny poly- 

 pidom closely resem- 

 bling that of the Hy- 

 drozoa. 



The Tunicata or Sea-squirts have no hard parts, but 

 are enclosed in a leathery tunic, which deserves mention 

 in this place on account of its remarkable chemical 

 composition, being composed of cellulose, a non- 

 nitrogenous substance which enters largely into the 

 composition of plants, but which is hardly known to 

 occur elsewhere in the animal kingdom. 



In the Mollusca (with which I include the 

 Brachiopoda) the integument or mantle very often 

 secretes a calcareous shell which is considered to be a 

 calcified epidermis. It consists of a basis of animal 

 membrane, hardened by carbonate of lime. If the 

 carbonate of lime be dissolved away by an acid, a soft 

 flexible membrane is left, which exhibits a structure 

 similar to that of the shell. Those Mollusks which 

 have no distinct head, the Brachiopods and Lamelli- 

 branchs, are provided with a two-valved shell ; the 

 shell, however, presents marked differences in the two 

 classes. In the former the shell is always equilateral but 

 never equivalve, the valves are back and front, the 

 ventral valve being the larger, and frequently pro- 

 longed beyond the other into a beak perforated at the 

 end for the passage of a muscle by which the animal 

 attaches itself to a rock. In the Lamellibranchs the 

 shell is generally equivalve, but rarely equilateral, the 

 hinder end being generally produced. The valves are 

 right and left ; the valves are closed by one or two 

 powerful muscles, and opened by the recoil of an 

 elastic ligament and a cartilaginous cushion, so that 

 when dead the shell gapes. In the Brachiopoda the 

 shell is both opened and shut by muscular effort. The 

 Brachiopods have spiral fringed arms, which in some 

 No. 156. 



of the fossil forms were supported by slender watch- 

 spring-like processes of the shell. The structure of 

 the shell consists of flattened prisms which lie very 

 obliquely to the surface. In many of the species the 

 shell is perforated by numerous minute canals con- 

 taining prolongations of the mantle. 



In the Lamellibranchs, the shell has two layers, 

 the outer being made up of prismatic fibres, placed 

 nearly perpendicular to the surface, the inner, or 

 nacre, being laminated, and frequently reflecting 

 iridescent colours. The shells of the headed Mollusks 

 consist of a single valve, of a conical shape, and 

 generally spirally coiled ; in the GasterojDods, the 

 spiral is generally drawn out on one side, most fre- 

 quently the right ; in the Cephalopods, the spiral is 

 usually flat and symmetrical, as in the Nautilus. The 

 structure of univalve shells resembles that of the inner 

 layer of the bivalves. Some of the Gasteropods arc 

 without shells, others, as the slugs, have a thin shell 

 embedded in the mantle. 



In the higher Cephalopods, as the cuttle-fish, we 

 begin to find an approach to vertebrates in the pos- 

 session of an internal skeleton. The cuttle-fish bone 

 is well known : it is calcareous, and its structure is 

 very curious. It consists of flat plates parallel to 

 the surface, the interstices being occupied by flexuous 

 laminte at right angles to the others, like narrow 

 strips of corrugated iron cut in a direction across the 

 folds, and standing on edge between flat plates : this 

 form combines strength with lightness. The Belem- 

 nite had an.internal skeleton consisting of an elongated 

 guard of a radiating fibrous texture, pointed at one 

 end, and hollowed at the other into a conical cavity, 

 in which was lodged a chambered internal shell or 

 phragmacone ; at the upper end it was prolonged into 

 a horny wing-like expansion. 



We are now arrived at the highest or vertebrate 

 sub-kingdom. In this sub-kingdom, phosphate of 

 lime takes the place of carbonate as the hardening 

 material, the latter being seldom met with, though the 

 shell of the eggs of birds is an instance. The hard 

 parts of the body arc grouped into two systems, the 

 internal skeleton and the dermal skeleton. The 



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