Having entered into these details respecting the Asterias, I may describe more briefly the phenomena in the Sea-urchin, the more so as my opportunities of observing this animal have been less frequent.
The species submitted to examination was the common large Sea-urchin of our shores, Echinus esculentus, described by Monro.* Its body consists of a globular shell, containing the viscera. The mouth is placed underneath, the anus opposite on the upper surface. The tubular feet are disposed in vertical rows from the mouth to the anus, the intermediate part of the shell being covered with moveable spines, and the singular claw-like organs referred to in describing the Asterias. As in the Asterias, there are membranous respiratory tubes, but they are comparatively few in number, forming ten small bunches or groups, which are placed on the under surface not far from the mouth, and open internally in ten small perforated pits, like those of the Asterias; they are supposed by Tiedemann and others to be the channels by which the sea-water gets into the interior of the body, and fills the space between the inside of the shell and the contained viscera. The ali mentary canal, commencing at the mouth, rises through the curious dental apparatus named Aristotle's lantern, turns in a waving manner twice round the inside of the shell, and termi nates above at the anus ; it is supported by a mesentery derived from a membrane which lines the cavity of the shell, and which is reflected over its contents like a peritoneum. Inside the shell we also find the ovaries and the rows of feet. The internal parts of the latter, instead of being round vesicles as in the Asterias, are broad lamince enclosing vessels,t canals or branched cavities, which canals, like the vesicles of the Asterias, co-rnmunicate on the one hand with the tubes of the feet, and on the other with a common vessel which runs along the middle of each double row of lamime. The vessels or spaces within the laminw are much branched ; they form a plexus surrounded by a principal vessel at the border.
I have found the ciliary motion over nearly the whole surface of the cavity of the body and the contained parts, which surface, as mentioned already, is covered by a lining membrane or peritoneum. Two longitudinal currents run on the intestine in the same direction, viz. one along the line of attachment of the mesentery, the other at the opposite part of the tube. On the remaining circumference of the intestine the impulsion is directed obliquely towards the nearest longitudinal current. In regard to the
laminx of the feet, a current runs down the middle of each of the double rows, following the course of the longitudinal vessel there situated, the direction being from the anus to wards the mouth. Lateral currents pass over the surface of the lamina2 from their external to their internal border, where they join the middle current; they follow the irregular eleva tions on the surface of the laminze occasioned by the canals or vessels in the latter; hence, when charcoal pol,vder is applied, the particles follow winding paths in crossing from one edge of the lamina! to the other, and they are fre quently caught in a hollow between two cur rents, and whirled about for some time bePore they resume their way. Currents were visible also on the reflections of the lining membranes which cover and pass between different parts of the lantern, and at the internal openings of the respiratory tubes. The cilia on the parts de scribed are excessively small, but distinctly per ceptible. The ciliary motion was not detected on the external surface of the body nor within the alimentary canal ; but in regard to these parts the observations could scarcely be consi dered as conclusive; nor could I determine whe ther, as in the Asterias, the phenomenon occurs within the feet or within the spaces or vessels of their membranous laminw, though from an observation of Carus, who states that he saw globules circulating within these laminm, its existence in that situation is not improbable.* This provision in the Echinus is probably, as in the analogous cases already described, chiefly subservient to respiration. Tiedemann, who ascribed a respiratory office to the water within the animal, expresses himself at a loss to con ceive by what mechanism it can be made to enter and issue from a cavity with unyielding sides incapable of being expanded and con tracted by muscular action ; perhaps the provi sion here described may be adequate for this purpose. Since the above observations were made, a fact has been mentioned by Ehrenbergif from which it appears that the ciliary motion exists on the external surface of the Echinus on the spines. The species observed by him was the Echinus sexatilis. The observations of Carus and Ehrenberg here referred to comprehend the only facts hitherto published on the ciliary motions of the Echinus which have come under my notice.