USE OF CILIA IN RESPIRATION. 871 



branchiata, mussels, oysters, and others, the gills consist, as their name 

 implies, of two pairs of flat lamina, composed of a double layer of membran- 

 ous rods, covered with fine cilia, and supporting the bloodvessels. In some 

 genera, those branchiae are freely exposed when the valves of the shell are open 

 (oyster); in others, they are inclosed by the ciliated lobes of the mantle, at 

 each end of which an aperture exists, for the entrance and exit of the water, 

 which is driven through by ciliary action. 



Molluscoida. Amongst these, which are all aquatic, the Ascidioida possess, 

 in the anterior part of the body, large respiratory atria or chambers, or 

 branchial sacs, lined with cilia, and communicating with the exterior. These 

 atria are formed by an expansion of the part formerly named the pharynx, 

 which is supported by a rod, called the endostyle, the sides of which are, in 

 some species, cleft by numerous slits, through which water passes into an 

 interval b3tween the branchial atrium and the proper walls of the body, and 

 then escapes at the opening of the mantle. In the Brachiopoda, the cavity of 

 the mantle itself is the chief respiratory chamber ; it presents vesicular dila- 

 tations covered with vessels, which probably act as gills. In the Polyzoa, the 

 perivisceral cavity, which contains nutrient fluid, is prolonged into the nu- 

 merous, delicate, tentacular processes, arranged around the oral aperture, which 

 are covered with rows of beautiful cilia; hence the fluid in their interior is 

 aerated by the currents of water on their surface. 



Annulosa. The aquatic Annulosa are represented only by the Crustaceans, 

 and the Tardigrade Arachnida. In the larger Crustacea, the gills or branchiae 

 present the form of flattened laminae, which, in the higher forms, such as the 

 lobsters and crabs, are inclosed in proper branchial cavities within the shell; 

 in the lobster, there are twenty-two branchiae on each side. Through these 

 cavities, copious streams of water are propelled by the continual movements 

 of special flapping organs, consisting of the last joints of some, or all, of the 

 abdominal limbs, which are flattened out for that purpose. 



In the land crabs, which are drowned if kept in water, but which remain on 

 land a long time, the gills are moistened by the watery contents or secretion of 

 a spongy or laminated organ, situated, with the gills, in the gill-chamber. In 

 still lower and smaller Crustacea, the branchiae consist of delicate foliaceous or 

 flabellar organs, attached to the under side of the segments of the body, which 

 project into the water, and are usually kept in a constant state of motion. In 

 the Crustacean Onisci, or wood lice, and allied forms, which inhabit moist 

 places, there exist, near the covered gills, networks of air-tubes, or even air- 

 sacs, thus approaching the Insects. In some of the lowest aquatic Arachnida, 

 as the Pycnogonida, no special respiratory organs have been detected. No 

 cilia exist in any of the aquatic Arthropodous Annulosa. It has already been 

 mentioned that the larvae of certain Insects which live in water, breathe by 

 gill-like appendages connected with the abdomen, or with the intestinal canal. 

 The Annelida or worms are generally branchiated; the gills are composed of a 

 highly-divided delicate membrane, either branched or tufted, and usually cili- 

 ated; they are sometimes external, and situated either on the back, on every 

 segment of the body, as in Nereis and Eunice, or around the head only, as in 

 Serpula; sometimes they are internal or covered, as in Polynoe; or they are 

 represented merely by ciliated sacs, as in the leech. 



Annuloida. These are, also, either aquatic, or parasitic in the interior of 

 other animals. Such as have soft integuments probably respire through the 

 skin. None ever possess branchiae ; but special provisions for the respiratory 

 function are met with. Thus, in all the worm-shaped Scolecida, there exist 

 peculiar ramified contractile vessels, the trunks of which open on the surface 

 of the body, and are in part ciliated in their interior ; these are the so-called 

 water-vessels, which are supposed to be subservient to the respiratory process. 

 In the Rotiferous animalcules, ciliated discs of the most varied shape perform 

 this function. In the Echinodermata, respiration seems also to be performed, 

 partly by the sides of the perivisceral cavity which communicates generally 

 with the exterior, admits the sea-water, and is lined with cilia ; and partly by 

 the contractile ambulacral vessels, which can also be distended with fluid. In 

 the Holothurida,- the perivisceral cavity is closed ; the ambulacral system is 

 reduced to its lowest condition, but important ramified tubular organs, com- 



