MKMOIHS OK TMK NAT1()^'AL ACADEMY OK ^('IKNCKS. 



:}I7 



In the Mciiistoniatii there must liave been a coiisithM-alile diversity of shape and mode of 

 attaeliment. Fortunately we know somethiuf,' of the form of those of 81iinonia, an Kurypterid. 

 Dr. Henry Woodward luis (h^serihed and li^rured them in liis valuable Monograpli of tlie British 

 Merostonuita (1*1. xix, Fij,'s. .{ and 4, ]>. 115). Similar ones in PtirytjoluH hiltihm \iii\c been fle- 

 scribed and lifjnred by him. (PI. Xil, V\g. hi). Woodward thus des(^ribes those of Slimonia: 



TIu'.v :ii)]>i-iir to hts urraiiKcil in liimar scries luid were iiltai-lii'tl in siuffld or doiibld rowH to llio under Kiirfiicf iil' 

 tlie b.idy by their miper en.l, whilst their lateral sin.l Iowit roiin.lrd leallike borders were freely bathed in the llnid 

 niediiim in imler to dXygeunte the bluod. 



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

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

 approached the sides (p. 115, 116;. 



These delicate membranous plates in Pterygotus ditt'or in form from the corresponding organs in Liraulus; but 

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

 ference of the lamelbe; in Pterygotus they appear tu have branched and subdivided from the center to the margin 

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



III.STUKV OF OPINION AS TO TUK HOMOLOGY OP THE GILL BEARING ABDOMINAL LEGS OK LIM- 

 ULUS WITH THE BOOK LUNGS OF ARACHNID A. 



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

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

 abdominal ajipendages 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, 

 hoireiii; tJvrclopingfiom these) appear eight f/ill-poiiches (Kiemenhieher). 



Further on Metschnikotf describes the development of these lungs: 



The lungs form invaginations of the chitinous layer which are situated directly under (rfic/i/ nnter) the segment 

 appendages of the four abdominal segments (Taf. xvi. Fig. 12 p. n.). They appear first in the form of cup-shaped 

 sacks which open th<mgh 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 second pair [of appendages] of the same), they 

 become more roomy and deeper. But. first in the latest embryonal 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 e.vternal gill-opeuing becomes by this time consider- 

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

 8ei)aral;ed a fine cutieula. Moreover, hero and 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 oiigin and process of invag- 

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

 tory abdominal appendages under which they arise. 



" From a ta.Konomic point of view too much dependence must not be placed on the position and arrangement of 

 the gills or respiratory organs in any chiss. For example, in .Scutigera, a Myriopod, the trachea! are grouped into 

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

 cardium and thus .aerate the blood. (See Sinclair, ".\ new mode of respiration in the My riapods," Ann. and M.ig. Naf . 

 Hist. .March, 1X92. .See also Voges's Uas Respirations-System der Scutigeriilen. Zool. Anzeiger, V, I8S2, pp. 67-69. Also 

 K. Tomiisviiry, I'eber das Respirationsorgan der Scutigeriilen (Vorliiulige Mittheiliing) Math. Katurw. Ber. lugaru 

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



