68 Miss Agnes Crane — Evolution of the Brachiopoda. 



to that sub-section of spiny Eliynchonelloids denominatecl Acantlio- 

 thyris by D'Orbigny, so abundantly represented in the Oolitic seas. 

 That acute Norman palaeontologist, Eugene Deslongchamps, however, 

 considered it more closely reproduced the fashion of spinose orna- 

 mentation, characteristic of certain Palaeozoic genera, as the spines 

 are arranged in regular rows at intervals, as in Atrypa, a Palaeozoic 

 form, and ai'e not irregularly dispersed over the surface, as in the 

 spinose Rhynchonellae of the Jurassic epoch (34). 



There are, indeed, "all sorts and conditions" of Brachiopoda; 

 they vary in size from a pin's head to nearly a foot in length and in 

 breadth {Productus giganteus), from the Carboniferous Limestone. 

 Their familiar forms present so many queer shapes and ornaments 

 that it is impossible now to detail their external features. 



But something must be said with regard to the animal inhabiting 

 the shells, for the days of the simple conchologist, content with 

 mere externals, have passed away — or he survives only for the benefit 

 of the dealer. The study of what was scornfully termed the " nasty 

 animal " has become one of the most important conditions of bio- 

 logical research from the earliest free-swimming larval phases to 

 the adult fixed condition of the individual. The embryologist now 

 follows the life-history of a Brachiopod from the ova through all its 

 stages of development. The bathmologist watches it from the first 

 moment of the growth of the protegulum, or initial shell-covering, 

 through infancy to youth, from youth to adolescence, from adolescence 

 to maturity, from maturity to decline, and from decline to absolute 

 senility of extreme old age, when the venerable Brachiopod, like 

 more exalted organisms, again reverts to its infantile characteristics. 

 Then, alas ! it sheds its surface ornaments, becomes bald, so to speak, 

 and may even become obese ! 



It is to the recognition of the value of these methods of investi- 

 gation, or the twin sciences of embryology and auxology, or bath- 

 mology, as Hyatt prefers to term it, inaugurated by Heckel, of Jena, 

 extended by Hyatt, of Boston, and systematically applied to the 

 Brachiopoda by Beecher and Clarke, that in a measure we owe the 

 revolution of thought concerning the evolution of the Brachiopoda. 



The class is again subdivided into two sub-classes distinguished 

 by certain divergent features of both shells and animals, yet pos- 

 sessing some points of the nervous, circulatory, muscular, and 

 reproductive systems in common. The animal in both sub-classes 

 — developed from ova discharged from the fringed margins of the 

 mantle into the sea-water — begins life as an active free-swimming 

 ciliated embryo. It develops "eye-spots" and a peduncle by which 

 in many species it becomes attached by its hinder body-segment, 

 henceforth adopting a sedentary life. The shell, which is secreted 

 by the muscular " mantle " envelope, then makes it appearance. 



Brachiopoda belonging to the simpler and somewhat older type 

 are characterized by hingeless and often impunctate shells in which 

 phosphatic elements usually preponderate, with valves kept in place 

 solely by shell-muscles. The "arras" or breathing organs are of a 

 cartilaginous nature exclusively. The circulatory system is rudi- 



