205 



ARACHNIDA. 



d 



'have an apparatus for circulation pretty well 

 developed. It consists of an elongated vessel 

 placed immediately beneath the integument 

 along the middle line of the dorsal aspect of 

 the back, on which account it has received the 

 name of dorsal vessel (jig. 89). It is kept in. 

 its situation by small ligaments or muscles, 

 (a a), which in insects are called alts cordis. 

 The texture of the dorsal vessel is membranous, 

 and pretty firm ; it contains a colourless fluid. 

 This heart is in communication with numerous 

 vessels, but hitherto it has not been discovered 

 which of these terminate in, or which arise from 

 the organ, or, in other words, it is not known 

 by what route the blood arrives at, or proceeds 

 from the heart. We believe that we are able 

 to dissipate the doubts which still exist as to 

 this subject, but before we state our opinions 

 we shall speak of the anatomical disposition of 

 the apparatus. Treviranus has described it 

 vaguely in the scorpions, but has well elucidated 

 its structure in the spiders (araneee), more par- 

 ticularly in Club tone atrox and Tegenaria do- 

 mestica, Fig. 89. In both these species uu- 

 merous vessels are 

 observed to arise 

 from the heart, es- 

 " pecially from its 

 posterior part (c c.) 

 These proceed to 

 a ramify indefinitely, 

 d distributing them- 

 e selves over every 

 organ ; and we 

 a have no doubt 

 e with respect to 

 d their true arterial 

 nature. But in ad- 

 dition to these ves- 

 sels there exist two 

 o others of larger 

 a size (d d,*) which 

 communicate in 

 a one direction with 

 a the heart, in an- 

 other, by very fine 

 a ramifications, with 

 the pulmonary 

 branchiae. In Clu- 

 bione atrox these 

 b two vessels do not 



Tegenaria domestica. 

 branches in their course, 

 in our mind but that these vessels maintain a 

 direct communication between the heart and 

 respiratory organs. The subjoined figure 

 (Jig. 89) will facilitate the understanding of 

 these facts. It represents the heart and its 

 appendages in the house-spider, (Tegenaria 

 domestica,) and shows the two canals which 

 communicate with the heart and receive the 

 small vessels (e e ee) that come from the pul- 

 monary branchiae. Treviranus, to whom we 

 owe these observations, has not, however, at- 

 tempted to explain the manner in which the 

 circulation takes place in the arachnidans, and 

 indeed this is to be determined by physiolo- 

 gical experiment and riot by the dissection of 

 the organs merely. The experiments which I 



give out any 

 No doubt remains 



have made, in conjunction with my friend M. 

 Milne Edwards, on the circulation of the crus- 

 taceans, enable me to give a satisfactory and 

 doubtless true explanation of that of the arachni- 

 dans. The organs which exist in these animals, 

 and we admit the precision of the anatomical facts 

 detailed by Treviranus, are essentially the same 

 as in the crustaceans. We find a heart, of the 

 nature of which no one can entertain a doubt : 

 then there are arteries proceeding from the 

 heart and ramifying over every part of the 

 body; lastly, the heart receives on each side 

 vessels which bring it into communication with 

 the respiratory organs. These latter vessels are 

 the analogues of the branchio-cardiac vessels 

 of crustaceans. With respect to veins, of which 

 the latter animals are destitute, they are equally 

 wanting in the arachnidans, and are doubtless 

 replaced by cavities of an irregular form which 

 exist between all the organs of the body. Tre- 

 viranus, indeed, has remarked in the abdomen 

 of Tegenaria domestica two small intervals 

 which are discoverable through the integument, 

 and in which he says the blood may be ob- 

 served to be collected. These reservoirs are 

 perhaps the analogues of the venous sinuses of 

 the Crustacea. 



The nature of the vessels being thus deter- 

 mined, it becomes easy to conceive how the 

 circulation takes place in the arachnidans 

 the blood, leaving the heart, is distributed 

 through all the arteries to the different organs 

 for their nutrition : this being effected, and the 

 nutrient fluid being thereby converted into ve- 

 nous blood, it begins to circulate through the 

 sinuses before mentioned, and arrives by an 

 insensible course at the pulmonary branchiae. 

 There it is changed by contact with air into 

 arterial blood, and returns to the heart by 

 means of the branchio-cardiac vessels (e d} to be 

 finally again propelled through the arteries (c.) 



Thus the ascertained anatomical facts, few 

 as they are, permit us already to appreciate the 

 mode of circulation in the arachnidans ; and we 

 repeat that it is in every respect analogous to 

 the circulation in the crustaceans. 



Nervous system. The nervous system is 

 gangliated, as in all the articulate animals; 

 but it presents considerable differences of dis- 

 position in the different arachnidans : the 

 scorpions in this respect vary much from the 

 spiders. 



In the Scorpionidte we find the following 

 structure (Jig. 90): the first ganglion, which 

 is commonly called the brain (a), and which 

 supplies the nerves to the parts of the mouth (6,c) 

 is intimately blended with the nervous mass 

 giving origin to the nerves of the legs (d). The 

 succeeding ganglia are distinct from one an- 

 other, and are seven in number : the Jirst three 

 (1 2 3) are situated in the abdomen proper; 

 they have this peculiarity, that they are united 

 together and with the ganglion, which may be 

 termed cerebro-thoracic, by three instead of 

 two chords of communication (e), which is the 

 number found in all other articulate animals; 

 the^bwr remaining ganglions (4567) occupy 

 the entire length of the post-abdomen, or that 

 contracted portion of the body which is incor- 



