RESPIRATION IN WORMS AND CRUSTACEA. 365 



ratory surface is maintained by the vibration of the cilia, with which 

 they are clothed. 



656. The position of the gills, in the Mollusca of higher organization, 

 is extremely variable. Sometimes they are disposed upon the external 

 surface of the body, and form delicate leaf-like or Fig 101 

 arborescent appendages (Fig. 101) ; whilst in other 



cases they are enclosed in a special cavity or gill- 

 chamber, to which water is freely admitted from 

 without; a continual interchange being provided 

 for, either by ciliary action, or by muscular move- 

 ments specially adapted for the purpose. The 

 blood is conveyed to them, after having become 

 venous in traversing the capillaries of the general 

 system, by means of large channels and sinuses 

 excavated in the several parts of the body ( 556) ; 

 and after being aerated in the gills, it returns to 

 the heart, to be again conveyed to the system. In 

 the Cuttle-fish tribe, there are supplementary ^Liard 8 * 0111 ' separated 

 hearts at the origin of the branchial arteries, or 

 vessels that distribute blood to the gills ; and these have evidently for 

 their purpose, to render the respiratory circulation more energetic, and 

 thus to increase the aeration of the blood, in the degree required for the 

 vigorous habits of these animals, which, present a remarkable contrast 

 to the sluggish, inert character of the Mollusca in general. In these 

 classes, taken as a whole, the respiration is low in its amount. The 

 blood contains no red corpuscles, excepting perhaps in the highest class ; 

 and the change in its composition, which is effected by the air, is con- 

 fined, therefore, to the fluid plasma, or liquor sanguinis. And as it is 

 not exposed directly to the air, except in a few species, but to the air 

 contained in the water inhabited by the animals, this change cannot be 

 very energetically performed. But as the life of these animals is chiefly 

 vegetative, as their movements, except in the highest classes, are few 

 and feeble, and as they maintain no independent heat, there is but 

 little need of that interchange, which it is the object of the respiratory 

 process to effect ; and these animals can sustain the complete suspension 

 of it for a long time. 



657. Among many of the Articulated tribes, the respiration is car- 

 ried on upon a similar plan. In some of the lowest, such as the Tape- 

 worm of the intestinal canal, there is no special provision for the aera- 

 tion of the fluids ; the soft integument permitting the extrication of car- 

 bonic acid, and imbibition of oxygen, in the required degree. This is 

 but very small, however; the life of these animals being almost purely 

 vegetative. In the Marine Worms, which constitute a numerous and 

 interesting group, endowed with considerable locomotive powers, and 

 leading a life of almost constant activity, there is, on the other hand, a 

 special provision for this function ; the blood being transmitted, in the 

 course of its circulation, to a series of gill-tufts, which are composed of 

 a delicate membrane prolonged from the external surface of the body, 

 and which sometimes have the form of branching trees, and sometimes 

 of delicate brushes made up of a bundle of distinct filaments. In either 



