342 Dr. Shatpey on the Respiratory Currents. 



The foregoing are the principal facts I have as yet made out. 

 Of the cause which more immediately gives rise to the phenomena, 

 I am not prepared to offer any satisfactory explanation. It might 

 be supposed that, in most of the invertebrate animals, the motion 

 in the fluid is produced by the mere agitation of the cilia. The 

 operation of these, however, is probably not entirely mechanical, 

 because, though I have closely examined the gills of the tadpole 

 with a lens of less than the twentieth of an inch focal distance, I 

 have never been able to detect cilia on any part of their surface. 

 The currents, moreover, take place in the embryo of the Batrachia 

 at a period when the extreme simplicity of its structure renders 

 the existence of cilia highly improbable, and it is not likely that 

 phenomena, agreeing so much in other respects, are owing to diffe- 

 rent causes ; but whether they are to be referred to any of the 

 known properties of living bodies, or to a peculiar power residing 

 in the parts, is a question which in the meantime I would merely 

 submit to the consideration of physiologists, without venturing to 

 give any decided opinion. 



In conclusion, I may mention that most of these observations 

 have been several times repeated. The phenomena observed, are 

 for the most part sufficiently obvious, some being visible with the 

 naked eye, and almost all with a lens of moderate power, I have 

 shown them to several of my friends, among whom I may mention 

 Dr. Thomson, Mr. Syme, and Mr. Allen Thomson, to the last of 

 whom I feel much indebted for his obliging assistance in various 

 ways, in the course of these inquiries. 



EXPLANATION OF THE PLATE. 



Fig. I. The larva of the frog a few days after coming out of the egg. 

 Fig. 2. Magnified view of the same. 



Fig. 3. Plan showing the direction of the currents on a portion of the giU. 

 Fig. 4. The common mussel viewed from the right side ; the right half of the 

 shell having been removed, and the mantle turned down to show the parts within. 



a, a, a', a", The circumference of the mantle or cloak ; between a' and a" it is 

 fringed with tentacula or cirri, and it is nearly in this space that the water enters, 

 the borders being usually contiguous in other parts. 



b, The posterior muscle which closes the shell, partly concealed by the re- 

 flected edge of the cloak. Between a' and b is seen the opening by which the 

 water issues. 



c, c'. The two gills of the right side ; d, one of those of the left. The dotted 

 arrows mark the course of the water that has been forced tlirough the insterstices 

 of the vessels into the interior of the gill. The dark arrows indicate the direc- 

 tion of the external current, which perhaps serves also to clear the surface of the 

 gill from foreign matters, and convey nourishment to the mouth. 



e, e. The free edge of the unattached layer ; under it is seen the opening where 

 the water sometimes escapes from the interior of the gill, and through which a 

 probe can be passed between the two layers. 



/. The extremity of the foot, or the tongue as it is sometimes called. 

 ■ m, m. The two labial appendages or accessory gills of the right side ; w, one 

 of those of the left. Between them and before the foot is the opening of the 

 xnouth, which cannot be seen in the figure. 



For references to Fig. 5. and 6. vide " Natural-Historical Collections." 



