406 Dr. T. Williams on the Mechanism of Aquatic 



doned to themselves they may be likened to grains of sand, be- 

 tween which there is no cohesion. These few preliminary remarks 

 are made, at once, in apology and explanation of the plan of 

 investigation which the author has adopted in these papers. He 

 has sought at the expense of great labour, and heavy cost in 

 several senses, to amass such a store of minute facts as will con- 

 stitute at another time the ground of an appeal in support of a 

 wide generalization. With this explanation he will now proceed 

 to complete the serial history of the organs of respiration in 

 Invertebrate animals. 



In a former paper it was stated, in relation to the gills of the 

 Lamellibranchiata, that from the Tunicata to the highest mol- 

 lusk of this class, there prevailed a unity of structure which 

 acknowledged no single real exception. In every form of branchia 

 the blood-channels were straight, parallel, and non-communi- 

 cating ; that at the free border of the gill they returned upon 

 themselves in form of loops, and that thus the afferent layers of 

 vessels became separate and distinct from the efferent; that 

 whether these layers were two or four in number, the real 

 architecture of the organ remained unchanged*. 



These fundamental facts will be found to pervade every variety 

 of gill to be found among the higher class of the Gasteropod 

 and Cephalopod Mollusca. In fact, wherever there is a sepa- 

 rately developed branchia within the range of the Molluscan 

 subkingdom, these principles of organic construction will receive 

 an illustration. But between the Lamellibranchiate and Gaste- 

 ropod gill, several important and striking differences exist. In 

 the former the blood-channels are of large bore ; they are capable 

 of carrying a voluminous column of fluid : such a fact implies 

 that in the organ of respiration in this class the fluids are not 

 minutely multiplied and subdivided, and that consequently the 

 contact between them and the external aerating medium is less 

 intimate and complete. Another interesting fact to be noted is, 

 that the corpuscles of the blood of the Lamellibranchiata are 



* In the paper to which I refer, I promised to show that the apparently 

 rectangularly- arranged blood-channels of the branchiae of the Tunicata 

 were not real exceptions to the rule stated. In the collected volume of 

 these memoirs, it will be proved beyond doubt that the ultimate vessels 

 in the pharyngeal gills of these inferior mollusks are disposed in " parallel, 

 straight, non- communicating order," and that the crossing which takes place 

 between the larger and the smaller blood-channels is a mere appearance 

 depending upon the mode in which the gill is folded in the cavity. It will 

 then also be shown that the ultimate blood-vessels in the branchiae of the 

 Tunicata are bounded by hyaline cartilages which also define the channels, 

 precisely in the same manner as is done by the corresponding cartilages in 

 the gills of the higher Lamellibranchs. 



