CRUSTACEA. 



7f7 



The blood returns from tlie different parts of 

 the body by canals, or rather vacuities among 

 the tissues, (for they have no very evident 

 appropriate parietes,) which terminate in the 

 venous sinuses, situated close to the branchiae. 



In the short-tailed Decapoda we find no 

 more than a double series of these sinuses, 

 included within the cells of the flancs above 

 the articulations of the extremities. They com- 

 municate with one another, and they appear to 

 have no parietes other than laminae of cellular 

 membrane of extreme tenuity which cover the 

 neighbouring parts. Each of them, neverthe- 

 less, receives several venous conduits, and 

 gives origin at its superior and external part to 

 a vessel which, traversing the walls of the 

 flancs at the base of the branchiae, conducts 

 the blood to the latter organs. This is the 

 external or afferent vessel of the branchiae. 



We find the same lateral venous sinuses in 

 the Macroura; but instead of communicating 

 with one another athwart the thoracic septa, as 

 is the case in the Brachyura, they all empty 

 themselves into a great median vessel, which 

 is itself a venous sinus, and occupies the 

 sternal canal. In the Squilla this sinus is al- 

 most the only vessel which serves as a reservoir 

 to the venous blood. 



The blood, after having been arterialized in 

 its passage through the capillaries of the 

 branchiae, is poured into the efferent vessel, 

 which, as we shall immediately have occasion 

 to see when treating of the respiratory process, 

 runs along the internal surface of each bran- 

 chia. It enters the thoracic cells in the same 

 manner as the afferent vessel passed out from 

 them, bends upwardly under the vault of the 

 flancs, and thus takes its course towards the 

 heart. It is to this portion of the canal that 

 we have given the name of branchio-cardiac 

 vessel. 



The mode in which the blood enters the 

 heart is still a subject under discussion. Our in- 

 quiries lead us to believe that this fluid, poured 

 by the branchio-cardiac canals into a sinus 

 situated on each side of the heart, penetrates 

 this organ by means of certain openings situated 

 in those parts of its substance which are 

 directly opposite to the canals mentioned. 

 But Messrs. Lund and Strauss imagine that 

 the blood is effused as it were into the peri- 

 cardium (which is named auricle by the latter 

 anatomist) to penetrate from thence by open- 

 ings situated on the superior surface of the 

 heart.* These openings, however, we conceive 

 to be closed in the natural state by means of a 

 membrane, and it is also worthy of remark 

 that the writers just cited were unacquainted 

 with the lateral openings which establish a 

 much more direct communication with the 

 interior of the organ. We must also add that 

 the celebrated John Hunter, whose labours 

 upon this subject have hitherto remained un- 

 known to the world, but which have very re- 

 cently been given to the public by Mr. Owen, 



had long ago ascertained the existence of the 

 venous sinuses and of the lateral openings of 

 the heart, although he seems to have thought 

 that the circulation was not complete in the 

 manner we have described it.* 



In the most inferior groups of the class of 

 Crustaceans the apparatus of the circulation 

 becomes much less perfect, and even seems to 

 disappear entirely in the last of the Haustel- 

 late tribes. In the Argula, for instance, there 

 still exists a heart, but the arteries as well as the 

 veins appear to be nothing more than simple 

 lacunae, formed in the interstices between the 

 different organs; and in the Nicothoa, &c. no 

 distinct trace of any portion of a circulatory 

 system has yet been discovered. 



C. Of the respiration. The Crustacea, like 

 all the other tribes especially formed for living 

 under water, respire by means of certain parts 

 of their external covering modified in its struc- 

 ture in order to fit it for this function, and 

 known under the name of branchi<e. This 

 character is even so completely inherent in the 

 organic type proper to this class, that it is still 

 preserved in certain species which live on the 

 land and not in the water. 



Nothing, however, can be conceived more 

 various than the form and disposition of the 

 organs of the branchial respiration among 

 these animals : in some the function is per- 

 formed by an extremely complex apparatus, 

 consisting in great part of organs created ex- 

 pressly for this end ; in others it is delegated 

 to certain appendages which do not exist for 

 the office exclusively, but are rather turned 

 from their more ordinary and obvious uses to 

 subserve this important function. In others 

 still, we neither discover special organs of 

 respiration nor other parts whose structure fits 

 them evidently to supply the place of branchiae; 

 in these cases we can only suppose that the 

 oxygen held in solution by the water acts upon 

 the nutritious fluid of the animal by the inter- 

 medium of the entire tegumentary covering. 



Let us first review the respiratory apparatus 

 in its state of greatest complexity, but com- 

 mencing with it in the embryo and following- 

 it in its progressive development, in order that 

 we may be the better prepared to compare it 

 with those forms which will be presented to us 

 among species less elevated in the series of the 

 Crustaceans. 



In the earliest periods of embryotic life of the 

 common Astacus fluviatilis, we discover no trace 

 of branchiae ; but at a somewhat more advanced 

 stage of the incubation, though still before the 

 formation of the heart, these organs begin to 

 appear. They are at first small lamellar appen- 

 dices of extreme simplicity, attached above the 

 three pairs of maxillary extremities, and repre- 

 senting the flabelliform portions of these limbs. 

 Soon these lamellar appendages elongate and 

 divide into two halves, one internal, lamel- 

 lar and triangular, the other external, small 

 and cylindrical ; lastly, upon the surface of 



* Lund, Doutes sur 1 'existence du systeme circu- 

 latoire dans les crustaces, Isis 1825. Strauss, Ainu. 

 comp. des Animaux articuies. 



VOL. I. 



* Catalogue of the Physiological Series of Com- 

 parative Anatomy, contained in the Museum of 

 the Royal College ot Surgeons, vol. ii. 



3 E 



