268 



NATURAL HISTORY. 



only are special to the Cambrian, twenty-nine to the Silurian, fourteen to the Devonian, nine to 

 the Carboniferous, while four are especially characteristic of the Permian strata which terminate 

 the Palaeozoic, or ancient life epoch, during which about 3,000 species existed. Some of these 

 occur only in certain localities, but many had as wide a geographical range in the past as some 

 representatives of the class in existing oceans. A species of the " spire-bearing " family, for 

 instance (Spirifer lineatus), is found in the Carboniferous rocks of North and South Europe, Asia, 

 America, and Australia. When the Silurian rocks were deposited, the Brachiopoda far exceeded 

 their bivalved contemporaries in number, but to-day the conditions are quite reversed, and the 

 Lamellibranchiata outnumber them in the ratio of forty-four to one. During the Mesozoic ages, 

 the Brachiopoda declined in numbers, and in the more recent Tertiary epoch their ranks became 

 sadly thinned. In the existing oceans they can be numbered only by tens instead of by thousands, 

 for but an insignificant remnant of 130 species now survives. They abound in all marine 

 deposits, and are most valuable guides in determining the relative ages of all the stratified rocks. 



The sexes are distinct in these bivalves, which are always reproduced by means of fertilised 

 ova, and never by the process of budding or gemmation. The eggs are white, kidney -shaped, 



and irregular, and are discharged in the form of a white powder 

 between the margins of the mantle, and hang in clusters from 

 its terminal fringe of bristles. They are deposited from April to> 

 September, and the animals attain their full growth in a single 

 season, and some, it is believed, live but a year. The embryo, 

 soon after its development from a circular cellular mass, becomes 

 subdivided into two, three, or four lobes or segments of unequal 

 size (Fig. 15, A). They are contractile, and swim freely by means 

 of the movable cilia with which they are covered. Eye-spots and 

 long tactile bristles are developed. Subsequently the embryo 

 attaches itself by the hinder segment, which forms the head of 

 the animal, the internal organs make their appearance, are suc- 

 ceeded by the mantle, and the rudiments of the shell plates, 

 which distinguish the adult animal. The embryos of existing 

 representatives of the earliest known hingeless forms, Lingula, 

 and Discina, are so closely allied as to be almost indistinguish- 

 able. The shape of the shell, and the length of the peduncle in 

 some members of the later-appearing family of Terebratulids, in 

 some stages presents a remarkable resemblance to Lingula, while, 

 at a more advanced stage of growth, the extreme prolongation 

 of the beak, or rostral portion, recalls the corresponding struc- 

 ture in the complex and widely distributed Silurian Trimerel- 

 lidae. At another stage, the hinge area resembles that of the 

 Spiriferae, and the young Terebratulina assumes the position 

 those genera must have adopted. 



Morse thus describes the actions of a young Terebratulina. 

 " The cirri of the arms moved frequently and in various 



directions, though generally performing a grasping motion, as if securing some bit of food, 

 imitating precisely the movements of the cirri in the Bryozca ; and this resemblance was 

 the more complete from the fact that the tentacles were densely clothed with cilia, and their 

 movements caused visible currents in the water." In a more advanced stage, the cirri (fringes) 

 stand erect upon the arms, and vividly recall the horseshoe-shaped forms among the Bryozoa. 

 In fact, the alliance between the Mantle-breathing bivalves and the Moss-animals is a very close 

 one, although at a first sight it must be admitted that no two animal types could apparently 

 be more dissimilar. 



The facts concerning the life history, distribution, and classification of the Brachiopoda, here 

 briefly epitomised, have been mainly derived from Davidson's well-known contributions to the 

 " Memoirs of the Palseontographical Society," the " Geological Magazine," and the " Results of the 



15. A, FREE-SWIMMING CILIATED 



LARVA OF TEREBRATULINA ; B, AT- 

 TACHED ; C, LATER STAGE ; D, HORSE- 

 SHOE STAGE OF LOOP. (After E. S. Morse), 



