MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



317 



In the Merostomata there muiSt have been a considerable diversity of shape and mode of 

 attachment. Fortunately we know something of the form of those of Slimoiiia, an Eurypterid. 

 Dr. Heniy Woodward has described and figured them in his valuable Monograph of the British 

 Merostomata (PI. xix, Figs. 3 and 4, p. 115). Similar ones in Pterygotus bilobus have been de- 

 scribed and figured by him. (PL xii, Fig. Id). Woodward thus describes those of Slimonia: 



They appear to bo iirrauged in linear series antl were attached in single or double rows to the under surface of 

 the body by their njiper end, whilst their lateral auil lower rouuiled leatlike borders were freely liathed iu the fluid 

 medium in order to oxygenate the blood. 



Fig. XXI. — A, gill-leaves of Slimou 



eparate one showing the "vascula 

 After Woodward. 



strijB ; " B, those of Pterygotus, both large and small. 



These leatlike appendages are highly vascular; they are about two inches in length and three-fourths of Tin inch 

 broad, but they vary in size, having probably been largest near the center of the body, becoming smaller as they 

 approached the sides (p. 115, IIG). 



These delicate membranous plates in Pterygotus differ iu form from the corresponding organs iu Limulus ; but 

 there is no doubt they occupied the same relative position. In Limulus the vascular striie are parallel to the circum- 

 ference of the laraellio ; in Pterygotus they appear to have branched aud subdivided from the center to the margin 

 of the plate, becoming finer and more numerous toward the border* (p. 68). 



HISTORY OF OPINION AS TO THE HOMOLOGY OF THE GILL BEARING ABDOMINAL LEGS OF LIM- 

 ULUS WITH THE BOOK-LUNGS OF ARACHNIDA. 



Nothing was said of the mode of origin of the book-lungs of the Arachnida until the appear- 

 ance of Metschnikofif's Embryologie des Scorpions (1870). He states: "The second pair of 

 abdominal appendages are transformed into the comblike organs, while the appendages of the 

 other segments of the abdomen totally disappear." In the place of the four pairs of the same (not, 

 hoicever, developing from these) appear eight gill-pouches (Kiemenlocher). 



Further on Metschnikoff describes the development of these lungs : 



The lungs form invaginations of the chitinous layer which are situated directly under {dUM unter) the segment 

 appeudages of the four abdominal segments (Taf. xvi. Fig. \2 p. «.). They ax>pear first in the form of cup-shaped 

 sacks which open though a broad mouth. After a further development of the lung-sacks, accompanying which there 

 is an atrophy of the abdominal segments (with the exception of the secoud pair [of appendages] of the same), they 

 become more roomy and deejier. But first in the latest embrj^onal stages (Taf. xvi. Fig. 14, from the ventral. Fig. 

 15, from the dorsal surface) there grows up out of the dorsal part of the lungs a blind tubular passage, whereby also 

 within the lung sacks begins the formation of the leaves. The external gill-opening becomes by this time consider- 

 ably smaller. The walls of the embryonal lungs consist of a cylindrical epithelium on whose inner side becomes 

 separated a fine cuticula. Moreover, here aud there on the outer surface of the lungs are certain groups of cells 

 which evidently belong to the mesoderm. 



Although Metschnikotf carefully describes and figures the mode of origin and process of invag- 

 ination of the book-lungs he does not entertain the idea that they are modifications of the transi 

 tory abdominal appendages under which they arise. 



* From a taxouomic point of view too much dependence must not be placed on the position and arrangement of 

 the gills or respiratory organs in any class. For example, in Scutigera, a Myriopod, the tracheiB are grouped into 

 structures like book-leaf tracheje placed iu a row down the median dorsal line. These lungs project into the peri- 

 cardium and thus aiirate the blood. (See Siuclair, "A new mode of respiration in the Myriapods," Aun. and Mag. Nat. 

 Hist. March, 1«S)2. See also Voges's Das Respirations-System der Scutigeridou, Zool. Anzeig(^r, V, 1882, pp. 67-09. Also 

 E. Tomo8v!try, Ueber das Respirationsorgan der Scutigeriden (Vorliiutige Mittheilung) Math. Naturw. Ber. Ungarn 

 I. Bd., p. 175-180, Taf. 4, Fig. 1-4, 1882-'83). 



