320 MEMOIKS OF THE NATIONAL ACADEMY OF SCIENCES. 



In Fig. 20 w(^ have a diagrammatic representation of tlie pulmonary sac of the Arachnids. The appendage (ga.) 

 has now become sunk in the body and the hole through which it passed is the stigma (stg.). The giU-lamelliB have 

 entirely disappeared and the pulmonary ones (pi.) have taken their place. The process here described is diflV^rent 

 from that imagined by MacLeod. It accords more with the development of the gills in Limulus and avoids the 

 necessity of union of the gill laminie and the expansion of the sternum. 



But the most convincing and direct proof of the conversion of a portion, not all, of the tempo- 

 rary embryonic abdominal limbs of the Arachnids into lungs has been afiforded us by tlie state- 

 ments and figures of Kishinouye in his essay "On the development of Araneiua," 1890. As will 

 be seen by the following extract, the view also suggested to us by Metschnikofl''s description and 

 figure is that the entire appendaife is not concerted into the hoo1;-Jung. Kishinouye also states 

 unqualifiedly that the third and tourtli pairs of abdominal appendages are transformed into spin- 

 nerets. His drawings (Fig. 34) directly confirm this and establish the truth of Salensky's suppo- 

 sition. 



In the l)asal part of the first abdominal appendage of each side there arises an ectodermic invagination, whose 

 opening faces away Irom the median line. It is neither deep nor spacious, but is a little pocketlike invagination. 

 This is the beginning of the lung-book. 



The development of this organ, briefly stated, is as follows: Of the wall of the invaginated pocket, that which 

 faces the distal end of the appendage is much thicker than the opposite wall, filling the interior of the appendage. 

 The cells composing it become after a while arranged in parallel rows (Figs. 34 and 47). Each two of tliese parallel 

 rows adhering together produce tlie lamellie of the lung-book. The external epithelium of the appendage which 

 cover these lamellfe becomes the operculum of the lung-book after it is depressed in height. Judging from figures 

 (Figs. LXXix and LXXix') given in his essay " On Insects and Arachnids," Bruce seems to have mistaken the caudal 

 prominence of the early period of this stage (see my Figs. 24-28) as the operculum of the lung-book. According to 

 him the abdominal appendage is invaginated to form the lung-book ; but as we have seen, it is not so. Locy has 

 correctly described the formation of the lung-book lamellie. He says that the lungs arise from infoldings; -but he is 

 silent about the place where these infoldings arise. 



In the basal part of the second abdominal appendage on the interior side, another ectodermic invagination is 

 produced. It assumes the shape of a deeply invaginated tube and remains in this condition till after the time of 

 hatching. The appendage itself is not invaginated and becomes from this time gradually shorter. 



It is very probable that the lung-books were derived from the gills of some aquatic arthropodous animal such as 

 Limulus, for the lung-books are nothing more than the lamellar branchiie of Limulus sunk beneath the body sur- 

 face. The tubular trachea may afterwards have been derived from the lung-books. The branchial lamellfe of Limu- 

 lus are formed as outgrowths of the ectoderm at the lower (posterior) surface of abdominal appendages, and those 

 of spiders are also produced really in the lower surface of the first abdominal appendage (in the dipneumonous 

 spider). Hence, I think that the spider with two pairs of lung-books is the most primitive one, and the one with 

 one pair of lung books and the other pair transformed into the tubular trachea; is more primitive than the spider 

 with only one pair of lung-books. I cannot agree with the view of some authors who maintain that the lung-book 

 is derived from a cluster of tracheie. 



The third and fourth pairs of the abdominal appendages are modified into spinning mammillie (PI. XV, Fig. 34). 

 At the distal end of each of these appendages a solid proliferation (ap.gl.) of ectodermic cells is formed. This becomes 

 the spinning gland. Spiders have generally three pairs of spinning mammilhe, two of which are modified abdominal 

 appendages, while the remaining one is added very late, after the hatching of the embryo. The primitive spider 

 must have had only two pairs of spinning mammillae. Some tetrapneumonous spiders have only two pairs. 



Laurie (1892) accepts MacLeod's thory "in its main lines," but suggests that "the lung-books 

 of the Arachnids were ij'robably derived from a series of paired plate-like appendages, not united in 

 the middle line by a gradual fusion of their edges with the abdominal walls of the body." 



Bernard (1892) thinks the fascinating and seductive theory of MacLeod and others will have to 

 be " given up in view of the great morphological difficulties which they involved" (Spengel's Zool. 

 Jahrbuch, 1892), the main reason he gives being that in Galeodes it is diflScult to believe that the 

 thoracic and abdominal stigmata had a separate origin. 



If now we return to Metschnikoff's description and figure of the mode of origin of the book- 

 lung of the scorpion, it will be seen that, as he states, the stigmata arise directly under the temporary 

 leg, as seen in his Fig. 12, Taf. XVI. The invaginated jxntion, then, does not represent the entire 

 leg, but a part of it, possibly the epipodal, which represents the branchial or outer division of the 

 gill-bearing leg of Limulus. The inner portion of the scorjjiou's leg then disappears by absorption. 



THE DEVELOPMENT 'OF THE BRANCHIAL LEGS OF LIMULUS. 



Our own observations on the branchial legs are ft'agmentary and have been made on embryos 

 of a stage nearly that represented by Fig. 20, PI. IV, of our first memoir, this stage being marked 

 by the appearance of the third pair of abdominal limbs. 



