Respiration in Invertebi'ate Animals. 303 



on its return to the proximal extremity. They are admirably 

 fitted for the intended purpose. 



The respiratory organs in the Decapod Crustaceans manifest 

 the highest specialization. They are fixed to the sides of the 

 thorax, and lodged artfully in expressly provided thoracic 

 chambers. 



Two types of structure prevail among this class, the tubular 

 or cylindrical, and the laminar. Both forms are exemplified in 

 the Macrourous decapods. The tubular is less common than 

 the laminar. Scyllarus, Palinurus, Gebia, and Homarus are 

 genera which afford examples of the cylindrical or tubular ; in 

 Astacus the tubules of the branchiae are less numerous, and are 

 disposed only on two of the sides of the branchial shaft, having 

 a pinnate appearance ; those tubules which are inserted on the 

 coxae are terminated by a thin, multiplicate, lamelliform dilata- 

 tion, and resemble in structure an ordinary branchial lamella. 



The coxae in Homarus and Palinurus are provided with a 

 plate-like process inserted at the side of the coxal branchia. It 

 is of a leathery consistence, and covered with numerous hairs. 

 The presence of these bristles proves that it cannot participate in 

 the process of respiration. It is probably only a septum of sepa- 

 ration between the branchiae. In Aristeus, in which the branchiae 

 are sixteen in number on either side, they are penniform. They 

 are composed of a shaft, from which pass off right and left 

 numerous curled filaments, whose convex borders are covered by 

 tufts of very delicate, densely packed, branchial cylinders. 



The second or lamellar type of branchia occurs in all the Bra- 

 chyura and Anomura, and in the genus Galathea among the 

 Macroura. This type prevails also in the genera Palamon, Hip- 

 polyte, Alphaus, Penmis, Crangon^. 



The familiar Lobster affords the best example of the first or 

 cylindrical type of branchiae. They consist of plumose pyra- 

 midal processes, enclosed in a thoracic cavity, and provided with 

 peculiar flabellce. The latter subserve the twofold use of agi- 

 tating the water and cleansing, and separating the minute 

 tubercles of which the organs are composed. In number the 

 branchiae vary in different species. They amount to 20 in Asta- 

 cus, and in the most nearly allied species. In other Macroura 

 the number falls. In the Palinuri, Scyllari and Peneeus it is 18 ; 

 in Pandalus, 12; in the Calianassa, 10; in the Palcemons, 8; 

 7 only in the Crangons, Hippolytes, and Sergestes. In the 

 Lobster, the Crawfish, in Nethrops, Palinurus, and the Scuyllarus, 

 the branchiae are subdivided most elaborately into minute tubular 



* Ann. d. Sc. Nat. xi. 1827, pi. 26, and xi. 1839, pi. 3. fig. 1, pi. 4. 

 figs. 1-4. 



