DE. T. DAVIDSON ON EECENT BEACHIOPODA. 227 



" The changes undergone by the larva during development are gradual, and do not 

 involve any marked metamorphosis. ... In tlie figures, that valve of the shell which is 

 uppermost, is to become the movable valve of the adult : that wliich conchologists 

 have agreed to call the dorsal valve ; while the peduncle is to be attached to the ojiposite 

 valve, which is accordingly ventral. The nearly straight margin of the shell is that from 

 which the peduncle is subsequently to project, and therefore indicates the posterior end 

 of the body, while the opposite rounded margin is anterior." Prof. Brooks then proceeds 

 to explain the changes in the form of the larva, and adds : — " Before I pass to the de- 

 scription of the internal organization of the larva, I wish to call attention briefly to 

 the fact that the recent and fossil shells of various species of Crania, Discina, Lhigula, 

 Lingulella, Oholus, and other hingeless Brachiopoda, furnish a series of adult forms repre- 

 senting all the changes through which tlie outline of the shell of Lingula pyramidata 

 j)asses during its development." 



Prof. Brooks then describes the digestive organs, the body-cavity or perivisceral 

 chamber, the blood, the mantle and integument, the pallial sinuses, the muscles, the 

 nervous system and sense-organs, the peduncle, the lophophore and tentacles, and con- 

 cludes with an elaborate discussion of the bearing of the development of Lingula upon 

 the systematic position of the Brachiopoda. Indeed, the whole memoir is replete with 

 original and valuable details and suggestions, which have materially added to our 

 knowledge on many difficult and important questions*. 



* [An important memoir was i^ublished by Dr. H. G. Boyer " On the Structure of Lingula pyramulata ( = Glottidia 

 Andeharti) in vol. iii. no. 5, March 1880, of the ' Studies from the Biological Laboratory of Johns Hopkins University, 

 Baltimore.' 



Dr. Beyer considers the shell in this species to be composed of three layers, an outer periostracum or cuticle, " a 

 simple homogeneous and comparatively thiu layer or membrane. . . . probably a changed original larval ectoderm 

 or in some way produced by it." This cuticle covers the valves and the peduncle, becoming thickened and corrugated 

 at the point of attachment of the peduncle to the animal. Beneath this cuticle a large number of peculiar rounded 

 corpuscles are seen imbedded in the shell substance, sometimes in linear series, but generally scattered irregularly, or 

 aggregated in clusters. " Immediately adjacent to the cuticle and this layer of homogeneous round corpuscles, we 

 find a rather broad layer of horny substance." This varies in thickness according to the age and size of the 

 animal. Closely adjoining this broad horny layer a very thin calcareous layer occurs, and this alternation of horny and 

 calcareous layers continues through the shell. The horny layers, Dr. Beyer believes, partake of the nature of a sup- 

 porting substance, and " represent the homologies of the vertical septa found in the substance of the shells in 

 testicardine (articulated) genera. The calcareous layers probably are the result of a secretion on the part of the 

 former, or that of a calcareous degeneration of the ectodermal cells of that part of the mantle and body-wall wliich 

 is next the shell." 



The dorsal and ventral body-walls are closely adherent to and covered by the valves of the sheU, while the 

 remaining parts are comparatively free. The mantle is a fold of the body- wall itself. " The peduncle may properly 

 be looked upon as a worm-like backward jjrolongation of the body-wall and its cavity. The structural elements of 

 the body-wall, mantle and peduncle, are, first, an outer layer of ectodermal epithelium : secondly, a middle layer of 

 supporting tissue, variously modified according to situation ; and thirdy, an inner layer of lining or peritoneal 

 epithelium." 



Immediately beneath the ectodermal covering the calcareous plates are situated, more particularly distributed over 

 the inner leaflet of the mantle and the lateral body-walls. The supporting tissue, in its ordinary aspect, is a homo- 

 geneous layer of tissue either entirely structureless, or presenting a very faintly longitudinally striated appearance. 



All the so-called mesenteric bands are smiply bridges of a substance hitherto regarded as muscular, which also 



