Respiration in Invertebrate Animals. 85 



from the fixed border to the apex of the leaflet, that every single 

 spot of the flat surface of the latter is rendered available in the 

 active O|)cration of brcatiiing. 



Wlien a single lamina is detached and placed singly, floating 

 in salt water, under the microscope, and viewed as a transpai-ent 

 object, it may be supposed that the spectacle must be one of ex- 

 treme definedness, every one of whose constituent elements may 

 be readily singled out and read by the eye. This is an apriori, 

 and therefore as usual an erroneous fancy. Nothing is so diffi- 

 cult to the unpractised observer as to read clearly and accurately 

 the spectacle under view. It demands an exercised eye even to 

 distinguish an epithelial particle from a blood-corpuscle, a 

 blood-channel from the crease or fold of the lamina, a near ob- 

 ject from one placed at a greater focal distance. Practice and 

 perseverance will however enable the student to interpret with 

 confidence and accuracy all the subtle elements of this incon- 

 ceivably beautiful structure. 



A little experience in the art of viewing the branchial organs 

 of the Gastcropod Mollusks will suffice to assure the least inter- 

 ested observer, that the blood-channels traverse the plane extent 

 of the laminre in parallel vessels, of uniform diameter, separated 

 from each other by appreciable intervals, and bounded by indi- 

 vidual and independent walls (fig, 4 d, d). In Trochus they 

 appear to run (fig. 13 e) from the dorsal edge (a) to the free 

 border [b) along one face of the leaf, and back again along the 

 other surface, looping round the edge. On both surfaces they 

 are invested in a similar manner by ciliated epithelium, the cilia 

 being large at the edges and small over the flat face of the 

 lamina. 



Although the preceding account conveys an exact illustrative 

 image of the type which prevails throughout the branchial system 

 of this multitudinous order, yet as this occasion is the first on 

 which these particulars have been published, it is desirable to 

 enter into an examination of some few^ other examples of the 

 pectinibranchiate gill. 



In Phasiatiella the branchia is said to be partially detached 

 and free in its cavity, but in other relations it imitates the type 

 of the Trochidan organ. 



The Paludinidre are prosobranchiate gastcropod mollusks 

 which inhabit fresh water. It is curious to observe, that this 

 marked contrariety of habitat should occasion no variation of 

 place or structure in the organ of breathing. The branchia of 

 this family, like those of all piher Pectinibranehs, afiects a 

 position on the vault of the thoracic chamber, having the 

 rectum and generative ducts parallel to it on the right side, 

 and the mucous glands on the other. A siphon exists on the 



.3* 



