ARACHNIDA. 



205 



Fig. 84. 



gins of which adhere to the horny circle or 

 peritrema of the stigma before described. 



We here subjoin figures copied from those 

 of Professor Mu'ller of Berlin, which represent 

 these parts in a scorpion. 



Fig. 86. 



an infinite num- 

 ber of small 

 holes, between 

 which in the 

 centre we may 

 remark a larger 

 circular plate 

 (6.) Each little 

 aperture is as it 

 were stellated at 

 the margins (r,) 

 by which the 

 air penetrates 

 the body and 

 gets into the 

 t tracheae. These 

 tracheae are ana- 

 logous in struc- 

 ture and posi- 

 tion to those of 

 in sects; they are 

 elastic, ramify 

 after the man- 

 ner of vessels in the interior of the body, and 

 penetrate to even the minutest organs. 



With regard to the internal respiratory organs 

 of the Pulmonary Arachnidans they have a 

 very different character; presenting the ap- 

 pearance of membranous sacs formed by la- 

 mellse applied to one another like the leaves of 

 a book, each of these little receptacles opens 

 into a common cavity, the membranous mar- 



Ixodes Erinacei. 



Fig. 87 shows one of 



Fig. 87. 



Fig. 88. 



a 

 c 



i 



ddddddddd d d d 



the pulmonary branchiae entire, seen in profile : 

 a is the edge by which it adheres to the circum- 

 ference of the stigma; b the simple membrane 

 without folds ; c the folds or leaves. Fig. 88 

 shows a portion of the same pulmonary bronchia 

 laid open : a is the horny margin of the stigma, 

 or peritrema, to which the simple membrane b 

 adheres; c the common cavity into which each 

 pf the spaces opens which are formed by the 

 laminae. 



These organs resemble closely in their struc- 

 ture the branchial laminae, and hence Trevi- 

 ranus and Meckel compare them to branchiae. 

 Muller on the other hand maintains that they 

 are lungs, because, he says, they can be dis- 

 tended with air. The name of pulmonary 

 branchiae, which we have given them, seems to 

 reconcile the two contending opinions, although 

 we believe that the distinction between lungs 

 and gills is in itself of very slight importance 

 when applied to articulate animals. It is, for 

 example, quite impossible to establish such a 

 distinction in certain crustaceans, as the Onis~ 

 cus, the Asellus, the Cymothoa, which are all 

 provided with organs of an analogous structure, 

 although some live in water, and others in air 

 more or less humid. Moreover, certain crabs, 

 as the terrestrial species called Cancer Uca, 

 Ruricola, &c., of Linnaeus, possess branchiae 

 which are much better adapted for respiration 

 in air than in water. The Cancer Manas, so 

 common on our coasts, is almost in the same 

 case, since it passes a great part of its life out 

 of the sea, and it is well known that lobsters 

 and shrimps can live a long time out of water, 

 provided that the air in which they are kept is 

 humid. M. Milne Edwards and myself have 

 demonstrated, by decisive experiments, the 

 conditions in which the branchiae in these 

 animals act as lungs. 



Circulating system. The function of cir- 

 culation, which is always so intimately con- 

 nected with that of respiration, presents, as 

 might be supposed, two different conditions 

 in the arachnidans. Those which breathe by 

 means of tracheae have not an apparent circu- 

 lation ; and in this respect they resemble in- 

 sects : we attribute to them simply a dorsal 

 vessel without any ramifications. Those, on 

 the contrary, which possess branchial lungs, 



