186 Dr. T. Williams on the Mechanism of Aquatic 



pheral. In the Insect, as in the Myriapod and the Annelid, the 

 circulatory current acknowledges two main directions; in all 

 dorsal trunks, from the extreme tail to the antennae, the move- 

 ment of the blood is forwards, in all ventral trunks it assumes 

 an opposite course. 



In all branches connecting more or less transversely these two 

 systems of longitudinal trunks, the blood travels from below 

 upwards, from the ventral in the direction of the dorsal trunks. 

 On the supposition that the " systemic arteries" are distributive 

 neither in Insects nor in Myriapods, is it possible to solve the 

 mechanical problem of the circulation ? As now explained, the 

 principle of the system is intelligible and consistent. It is 

 asserted with great confidence by the physiologist, that in Insects 

 no distinction into venous and arterial blood can prevail. The 

 ubiquity of the tracheae renders such a distinction impossible. 



Such are the doctrines now taught; they aie not necessarily 

 true; at present neither the extreme peripheric parts of the 

 blood-system, nor those of the tracheae are known. New ques- 

 tions, to be presently propounded, will prove that the material 

 conditions of the processes of nutrition and respiration in the air- 

 breathing Ai'ticulata are as yet by no means clearly defined. 

 But let the central parts of the circulation in Arachnids be first 

 defined. 



To Mr. Newport is due the credit of first solving the problem 

 of the ' circulation^ in the Arachnid ; but his solution is neither 

 clear nor complete*. On his interpretation, conceding the merit 

 of correctness to his descriptive anatomy, the circuit of the 

 blood cannot be consistently described. Like that of the My- 

 riapod and the Insect, the dorsal vessel (fig. 3, p, a) of the 

 Arachnid extends from the tail to the head, along the dorsal 

 median line. With its sides, along its entire course, there are 

 connected two orders of branches ; first, the pneurao- or brancho- 

 cardiac canals (fig. 3, k, k, k), which return the blood from the 

 pulmonary or branchial sacs {g) ; secondly, the systemic arteries 

 {n, n) of Mr. Newport f. At its anterior extremity it breaks 

 forth into a great number of branches destined for the supply 

 of the appendages. To the dorsal vessel in the Arachnid Mr. 

 Newport assigns an extraordinary duplicity of action. According 



* See his papers in the Phil. Trans. Part ii. 1843. 



t I am very desirous in this place to invite the attention of the student 

 in comparative anatomy to the beautiful work now being published by M. 

 Eraile Blanchard ; it is entitled ' L' Organisation du Regne Animal,' a Paris, 

 chez Victor Masson et J. B. Bailliere. The dehneations which adorn this 

 work are executed in the first style of French ai-t. M. Blanchard impli- 

 citly follows the interpretation of the circulatory system of the Aj-achnids, 

 rendered by Mr. Newport. 



