Respiration in Invertebrate Animals. 261 



not aflfect the true blood ; its parietes are not supplied by blood- 

 bearing plexuses. The latter are remotely situated. To what 

 uses then is it dedicated ? It is surrounded by^ it floats in the 

 mass of the chylaqueous fluid. From their relative positions, it 

 is manifest that the fresh sea water admitted into the respiratory 

 tree either itself, or the air by which it is charged, passes by en- 

 dosmose through the partition of the parietes, and that the chyl- 

 aqueous fluid in the closed visceral cavity, either itself, or the 

 effete gases by which it becomes impregnated, passes out into the 

 respiratory tree by exosmose. This is the real function of the 

 ''respiratory tree of the Holothuria." It is an excentric appa- 

 ratus artfully provided, to renovate the composition and re- 

 plenish the volume of the chylaqueous fluid. Thus is pre- 

 sented a summary statement of the mechanism of respiration in 

 the Echinoderms. 



EXPLANATION OF PLATE XIL 



Fig. 1. Plan in outline of an Asteroid Zoophyte : a & d, visceral cavity or 

 space between stomach and exterior of the body in which the 

 chylaqueous fluid is contained ; b. shows the mode in which the 

 tentacles are supposed by some observers to terminate in and 

 open into the stomach itself; c, orifice at the bottom of stomach. 



Fig. 2. Plan of Hydraform Polype : a, base of tentacle opening into the 

 perigastric areolse c; b, stomach. 



Fig. 3. Actiniform Polype: b, visceral cavity; a, orifice at bottom com- 

 mimicating with this cavity ; c, tubular base of tentacle ; d, cilia 

 lining the interior of tentacle. 



Fig. 4. Plan of a Bryozoon : a, base of tentacle communicating with the 

 visceral cavity b. 



Fig. 5. Plan of Rhizostoma {Medusa) : 6, digestive sac ; d, c, gastro-vas- 

 cular canal. 



Fig. 6. Horizontal plan of the same : a, centre of digestive sac b; c,d, 

 gastro-vascular canals. 



Fig, 7. Vertical plan of a Ciliograde Medusan, Pleurobranchus : c, d, gastro- 

 vascular canals. 



Fig, 8. Section of an arm of Asterias : a, mouth ; e, opening from mouth 

 into the digestive caecum b; c, its further caecal end ; h, cavity or 

 body filled with the chylaqueous fluid ; /,/, membranous tubulai' 

 and ccBcal processes (the true branchia of the Starfish). 



Fig, 9. Vertical imaginary section of Echinus : a, mouth ; b, anus ; d, vis- 

 ceral cavity ; e, intestine ; f,f,f, hollow membranous processes — 

 the true branchiae of the Echinus ; g, suctorial processes. 



Pig. 10. Head and neck of Sipuncle : B, transverse section of one of the 

 branchiae, ciliated within and without ; C, the same, viewed trans- 

 parently ; i, coiled intestine ; h, corpuscles of chylaqueous fluid ; 

 m, j, blood-vessel ; n, visceral cavity. 



F^. IL A piece of the skin of Sipuncle showing the branchial fenestra 

 d,d,d: a, & pigmented epidermal cell in the centre of the unpig- 

 mented area b ; c, pigmented cells of the intervals between the 

 fenestrae. 



[To be continued.l 



