524 ORGANS OF RESPIRATION. 



and vents oxygenate every part of the exterior and interior of 

 their body. Vibratile cilia are also the agents of the respira- 

 tory currents in the adult and the embryo-state ofpolypiphe- 

 rous animals, where these minute organs are disposed along 

 the sides or around the external periphery of the tentacula, 

 or line their interior ; and they line the buccal and gastric 

 cavities of the polypi, and their prolonged canals which circu- 

 late nutriment through the body. In hydra and actinia, the 

 respiratory currents of water enter the stomach by the mouth, 

 and are seen passing to and fro within the ciliated cavities of 

 their tubular tentacula ; and indeed nearly the whole exterior 

 surface of the body and every internal organ, in the latter 

 genus, are closely covered with very minute vibratile cilia, 

 and are bathed by the aqueous currents of respiration. 



Although almost every kind of zoophyte is cilio-brachiate, 

 like actinia, and is amply provided with these minute organs 

 on the interior mucous lining of its alimentary apparatus, 

 the larger forms of symmetrically disposed brachial cilia are 

 generally confined to the lateral margins of the tentacula, 

 along which they move in very regular waves, following 

 always the same apparent direction, from the mouth of the 

 polypus on one side of the tentaculum, and towards the 

 mouth on the other ; but the direction in which these cilia 

 actually more is nearly vertical to the apparent plain of the 

 entire waves. The vibratile cilia are thus disposed on the 

 sides of the arms in most of the lower zoophytes, as alcyo- 

 nium, flustra, cellaria, serialaria, plumularia and sertularia, 

 which extend their elastic tentacula in a regular campanulate 

 form, while the ciliary currents flow towards the polypi, 

 aerating their surface and bringing food to their mouths from 

 a distance. These tentacula when severed from the body of 

 the polypi, continue to vibrate their cilia, and swim by their 

 action like worms through the water : thus showing, as in the 

 vibratile cilia of the mucous and serous membranes detached 

 from the body in all higher animals, the independence of 

 their movements on consciousness or volition, and their simi- 

 litude, in this isolated condition, to reflex or sympathetic phe- 

 nomena. 



The number of tentacula, like the number and size 

 of the cilia, varies much in different zoophytes, there 

 being but six tentacula in some hydrce, eight in plumularia 

 falcata, serialaria lendiyera, and many of the higher genera 

 as, lobularia, ptnnatula, virgularia, isis, corallium, and gor- 



