NATURE OF THE BRACHIOPODA. 



259 



ing this comparison with an Etruscan lamp. It is one, however, that is applicable to comparatively 

 few species. At one time these organisms were ranked with the higher shell-fish after the Gastero- 

 poda thus taking precedence of all the bivalve class. But they were not long allowed to maintain 



this position ; first reduced to a lower grade, they were afterwards ejected from the ranks of the 

 shell-fish proper, and relegated to an. inferior division of the molluscan type to which the name 

 of "mollusc-like animals"* was applied. Some recent authorities assign them a very different 

 position, and after detailed observations of the successive stages of development assumed by the 

 immature animals, place them with a section of the worm family (Annelida) that sin-round 

 themselves with a tubicolar sand-covering, a habit, by the way, which is shared by one family of the 

 Brachiopoda. But the adult animal of all the mantle-breathing bivalves is always enclosed between, 

 and protected by, an external shell forming two valves or pieces, which are generally regarded 

 as " front " and " back " shields, instead of right and left, as in the bivalve shells (p. 230). Each of 

 the two pieces composing the shell is always symmetrical in itself, but the shell is never " equivalve," 

 as one piece is invariably larger than the 

 other. Yet the expanded edges are 

 nearly always level and opposable, resting 

 one upon the other. 



None of the recent Brachiopoda are 

 very large, but many of the extinct forms 

 attained considerable dimensions, and in 

 one Carboniferous species (Prochtctus 

 giganteus) the shell sometimes measured 

 over a foot in length and in breadth. 

 The shells of many species are quite 



are 



smooth, or striated and marked with 

 circular lines of growth ; others are coarsely 

 or finely ribbed with longitudinal or 

 transverse ridges, or depressions. Some 

 are white and of a transparent and glassy 

 texture, as in the deep sea forms. Those 



inhabiting Arctic or Northern seas are of Fig 3 _ A> RHYNCHONELLA 8PISOSA> IXFERIOR OOLITE . M> ]1KODUCTUS 

 a dull grey colour, and the shell is more J.ONGISPINUS, CARBONIFEROUS ; c, CHONETES ; D, VENTRAL VALVE 



OF PRODUCTUS COMPLECTENS, LOWER CARBONIFEROUS LIMESTONE. 



(After Davidson and B.. Etheridge, Junior.) 



robust ; while the tropical species are often 

 brilliantly coloured, the prevailing hues 

 being crimson, yellow, emerald green, brown, or bluish black. Traces of coloration are occasionally 

 preserved in fossil specimens, many of which are beautifully sculptured externally, and some 

 extinct forms were additionally ornamented with long and elegant spines, attached to the outer 

 surface of their shells. In some genera the spines covered the whole exterior, and were coloured like 

 the shell, as in the Rhynchonella spinosa^ (A) of the Oolitic seas, or long and slender, were 

 irregularly distributed over the surface of the ventral valve (B). In others they were merely 

 developed in the region of the hinge (c). The spines, in one instance, armed with minute 

 booklets, were often four or five times the length of the shell. They were generally tubular, 

 sometimes with a double chamber like a gun. (Davidson.) Opinions have varied considerably with 

 regard to the functions of these appendages. Some writers consider them merely ornamental, 

 others, as canals admitting the sea-water to the interior of the shell, or as organs of attachment 

 which served as anchors to moor the animal at the bottom of the sea. A tiny species that 

 flourished in the Carboniferous seas, though a silent witness, furnishes irrefutable evidence that 



o 



they occasionally acted as clasping organs. Fig. 3, D, represents this small " embracing " Productus, 

 encircling the stalk of a sea lily (crinoid), with its slender spines. Thus the animal, enabled to 

 resist the rude buffets of the waves, was preserved from rough contact with surrounding objects. J 



The Brachiopoda are subdivided into two principal groups. All members of the first 



*The Molluscoida of Milne-Edwards. f Spiny little beak. A spine-covered species of this genus (Rh. dodcrlcini] has 

 been recently discovered living in Japanese waters. Eobert Etheridge, Jun., F.G.S., Journal of the Geological Soc., 187G. 



