252 Dr. T. Williams on the Mechanism of Aquatic 



cumference of the disc. Like those of the preceding families, 

 they are ciliated internally, while they are distributed in close 

 proximity to the under surface of the dome. 



The organization of the Physograde Medusa is little under- 

 stood*. 



It cannot be doubted, that in the fluids of the Acalephs, floating 

 corpuscles, from their multitude and their determinate sti'ucture, 

 exercise an important part. They animaUze the fluid; they 

 endow it with life. Directly or indirectly they develope the 

 proximate principles out of the inorganic elements. Both the 

 corpuscles and the fluid contained in the gastro-vascular canals 

 are nearly colourless. Here, as in many other instances amongst 

 the invertebrate animals, the lesson is taught, that colour has 

 little to do with the capability of vital fluids to absorb oxygen. 

 In this class, it is beyond question, that sea water is admitted 

 directly into those canals in which the chylaqueous fluid is con- 

 tained. The former is so rapidly assimilated with the latter, that 

 the nutritive and vital character of the compound fluid resulting 

 from the admixture is readily maintained at the required 

 standard. 



It is important to remark, that in all Acalephs the gastro- 

 vascular canals are distributed as closely as possible to some 

 external surface ; in Rhizostoma to the under surface of the 

 dome, in Beroe in meridional lines over the globe, in other 

 species along the margins, &c. Such disposition has reference 

 to the respiratory process : vibratile cilia in general are developed 

 on those portions of the external surface which coincide with the 

 gastro-vascular canals. 



Echinodermata. — In this class the same questions arise, as im- 

 portant preliminaries to the study of the respii'atory process, 

 with those as to the meaning of the fluids, which were discussed 

 with reference to the inferior Radiata ; — which of the three orders 

 of fluids, present in the oeconomy of nearly every Echinoderm, 

 is made the special subject of this process ? Jst, the cavity of the 

 body (figs. 8, 9, <:? ; fig. 10, n) {i. e. the spacious interval which 

 separates the digestive from the integumentary system) is filled 

 in all species with a fluid which the author has called the chyl- 

 aqueous : 2nd, the protrusile suctorial feet (PI. XII. fig. 8, g ; 

 fig. 9,/) are occupied by another class of fluid; this system 

 constitutes the water- vascular system of Tiedemann and Miiller ; 

 3rd, the blood-vascular system (fig. 10, y), of Tiedemann, Delle 

 Chiaje, Valentin, Agassiz, Dr. Sharpey and Miiller : these three 

 systems are defiined as severally distinct and independent, and 



* See a paper in the Ann. des Sc. Nat. torn, xviii. " Sur la Structure des 

 Physalies et des Siphonophores," by Leuckart. 



