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ordinary kind, and the other provided with the organs termed " lungs." The 

 present species was a good example of the latter class. The organ was 

 formed thus : the tracheae proceeded inward for a short distance, and then 

 expanded into a wide sac, the inner walls of which were so plicated as to 

 form a number of internal pockets, like the leaves of an uncut book, each 

 pocket communicating with a central chamber, which was joined to the 

 stigma by the short tracheal trunk before mentioned. Thus all the pockets 

 were full of air, and usually lay horizontally above one another, as though 

 the book were lying on its side. The blood entered the sac by special ostia, 

 and circulated freely between the leaves or pockets, becoming aerated in the 

 process. It was then carried forward by efferent pulmonary sinuses to the 

 pericardial sinus. Expiration was supposed to be effected by muscles passing 

 between the dorsal and ventral surfaces which compressed the leaves. The 

 course of the blood in spiders had been carefully studied by Claparede, and 

 was one of the numerous investigations which excited our wonder how that 

 naturalist, who died so young, managed to do so much and such accurate 

 work. He observed it in very young, living specimens, which were sufficiently 

 transparent to enable him to see the actual circulation under his microscope. 

 In insects the blood takes a backward course on the ventral surface, and the 

 circulatory system is there greatly lacunar, but the blood is driven forward 

 through the great dorsal, pulsating vessel. In spiders, however, Claparede 

 found that the course was different — the blood entered the dorsal vessel by 

 ostia near the centre, and was driven in both directions, some forward, some 

 backward ; so that there may be said to be two aortas, one anterior and 

 another posterior, the latter being the principal. It resulted from this 

 divided course of the blood that it was not the whole of the circulating fluid 

 that passed through the pulmonary sacs during each circulation, but only a 

 portion. Scorpions usually had eight pulmonary sacs ; spiders two or four 

 placed in the fore part of the ventral surface of the abdomen, In some in- 

 stances each sac might contain as many as 150 leaves. 



Mr. G. C. Karop enquired if the sac-like leaves figured by Mr. Michael 

 were open at both ends or whether at one end only, so that the blood cir- 

 culated between ? 



Mr. Michael said the latter was the case. 



Dr. Matthews asked if these lamellae presented any analogy to the gills of 

 fishes ? Also how did naturalists suppose that the entrance and exit of the 

 air was effected ? 



Mr. Michael said the lamellae were to a certain extent analogous to the 

 gills in structure, only they were not broken up into small leaves, and in 

 gills the blood circulated in closed vessels in an aerated medium, while in 

 tracheae it was the air which circulated in closed vessels. The only explana- 

 tion as to the inspirations and expirations was that the compression of the 

 tracheae by the muscles, and the telescopic retraction of the segments, would 

 expel air ; and when the pressure was relaxed, or the segments extended, it 

 would be drawn in. Others, however, had thought that it was done by a 

 double set of tracheae. 



