254 Dr. T. Williams on the Mechanism of Aquatic 



filled with a chylaqueous compound, under the unquestionable 

 form of a thickly corpusculated milky fluid, organized in a high 

 degree, and oscillating as a living nutritive fluid ; it is by track- 

 ing the characters of this fluid from above downwards, that its 

 real signification in the inferior Echinoderms, in which it offers 

 the apparent properties of simple sea water, can be unerringly 

 ascertained. The floating corpuscles of the chylaqueous fluid of 

 the Sipuncles (fig. 10, h) present the features of constancy in 

 structure and proportion ; they are always the same in the same 

 species. The cephalic appendages in this genus, as well as the 

 whole integumentary system of the body, are organized with ex- 

 press reference to the exposure of this fluid, and this fluid exclu- 

 sively, to the agency of the external aerating element. 



The skin m fenestrated (fig. 11, d, d, d), that is at regular in- 

 tervals the muscular layer disappears, and an interval of ellip- 

 tical figure, covered over only by a single layer of epidermis, 

 results. In the solid structures of the integuments there is no 

 trace whatever of a capillary vascular system to be detected. It 

 is a simple membrano-muscular partition, intervening between 

 the chylaqueous fluid within and the surrounding element with- 

 out : it is through this veil that these two divided fluids inter- 

 change their dissolved gases. The tentacles present the same 

 precise mechanism (PI. XII. fig. 10, a & C, & jB); they are merely 

 hollow appendages, musculo-membranous, lined within and with- 

 out by a ciliated epithelium. A few proper blood vessels reach 

 their bases from the circular vessel ; but no trace whatever of a 

 vascular plexus, in the structure of these parts, can by any 

 manoeuvre be discovered. The inference is irresistible, that, like 

 the skin of every part of the body, which internally is universally 

 ciliated, the tentacles are designed almost exclusively as instru- 

 ments for the oxygenation of the chylaqueous fluid {not the blood 

 proper), which oscillates by a flux and reflux movement in their 

 hollow interior. To the genus Holothuria these observations in 

 every detail are strictly applicable. The tentacles, however, 

 though hollow membranous appendages, are furnished, in the sub- 

 stance of their parietes, with a few more blood-vessels : the skin 

 is fenestrated like that of the Sipuncles (fig. 11) ; the open cavity 

 of the body is occupied by a highly organized corpusculated fluid 

 which the solid parts just described are expressly fitted to aerate. 

 From its volume, its organic composition and its suspended cells, 

 its importance in the organism cannot be disputed. It cannot 

 acquire nutritive properties unless through the agency of oxygen. 

 This element can be received through no other provisions than 

 those exhibited by the skin and the tentacles : thus the theory 

 of respiration, with respect to the chylaqueous fluid, in these 

 superior Echinoderms is complete. Although attenuated at 



