214 THE VITAL FUNCTIONS. 



these filaments. Organs of this, description are denominated 

 Branchiae, or Gills; and the arteries which bring the blood 

 to them are called the branchial arteries; the veins, which 

 convey it back, being, of course, the branchial veins. 



The larger Crustacea have their branchiae situated on the 

 under side of the body, not only in order to obtain protec- 

 tion from the carapace, which is folded over them, but also 

 for the sake of being attached to the haunches of the feet- 

 jaws, and thoracic feet, and thus participating in the move- 

 ments of those organs. They may be seen in the Lobster, 

 or in the Crab, by raising the lower edge of the carapace. 

 The form of each branchial lamina is shown at G, in Fig. 

 354:* they consist of assemblages of many thousands of mi- 

 nute filaments, proceeding from their respective stems, like 

 the fibres of a feather; and each group having a triangular, 

 or a pyramidal figure. The number of these pyramidal bo- 

 dies varies in the different genera; thus, the Lobster has 

 twenty-two, disposed in rows on each side of the body;, but 

 in the Crab, there are only seven on each side. To these 

 organs are attached short and flat paddles, which are moved 

 by appropriate muscles, and are kept in incessant motion, 

 producing strong currents of water, evidently for the pur- 

 pose of obtaining the full action of the element on every por- 

 tion of the surface of the branchiae. 



In the greater number of Mollusca, these important or- 

 gans, although external with respect to the viscera, are with- 

 in the shell, and are generally situated near its outer margin. 

 They are composed of parallel filaments, arranged like the 

 teeth of a fine comb; and an opening exists in the mouth for 

 admitting the water which is to act upon them.t In the 



* Page 193, of this volume. 



j- These filaments appear, in many instances, to have the power of pro- 

 ducing 1 currents of water in their vicinity by the action of minute cilia, similar 

 to those belonging to the tentacula of many polypi, where the same pheno- 

 menon is observable. Thus, if one of the branchial filaments of the fresh 

 water muscle be cut across, the detached portion will be seen to advance in 

 the fluid by a spontaneous motion, like the tentaculum of a polype, under 



