CILIARY MECHANISMS ON THE GILL IN AMPHIOXUS. 45 



■of recent researches on the function of the gill, however, it would appear 

 that the order of importance in which these functions have been regarded 

 must be at least reversed. The expenditure in the gill of the relatively- 

 large amount of energy necessary to produce the main current and food 

 •currents must enormously outbalance the aerating effect of these currents 

 on the gill. For it is to be remembered that in the gill-filaments of 

 Lamellibranchs the blood is not contained in a capillary plexus, but 

 usually in a blood vessel sunk well below the epidermis. This con- 

 clusion with regard to the function of the Lamellibranch gill has 

 indeed already been arrived at, from a morphological point of view, 

 by Dakin (6, pp. 52-3), who states that in the genus Pecten "It seems 

 certain, from the development of vessels in the mantle, that the great 

 function of the gills is to produce currents of water for aeration, nutrition, 

 and the carrying away of waste products, and the only parts of the gills 

 performing any really important duty in respiration are the branchial 

 expansions of the principal filaments." There is little doubt that this 

 statement might now be made similarly of the whole of the higher 

 Lamellibranchs. It is of interest, however, that there is one important 

 difference in this respect in the gills of the Protobranchia. In this group 

 the gill-filaments are broad and lamellate, and contain an extensive space 

 between the two surfaces (see Ridewood, p. 193) over which we have seen 

 the current passes. Doubtless with this type of gill a much greater 

 proportion of the whole respiration is effected in the gill than is the case 

 in the forms with narrow compact filaments. But even here it may 

 be doubted whether respiration is effected to the same degree in the gill 

 as in the mantle. It would be highly interesting in these respects if a 

 ready means could be found of detecting the relative amounts of waste 

 products in any particular part of the body at any particular instant. 

 Such an experiment may perhaps not be an impossibility, as some of these 

 animals can be observed living through a microscope. 



APPENDIX. 



Some of the observations made in the section on the function of the 

 Wheel Organ and Peri-pharyngeal bands in Amphioxus (p. 25) have 

 already been noticed by Andrews in the Bahamas Amphioxus, as will be 

 seen from the following quotation. It was unfortunately not possible 

 to refer to Andrews' work in the general text, as some difficulty was 

 experienced in obtaining his paper, which only became available after 

 the text had gone to the printer. 



Andrews ( 4) obtained living specimens of the Bahamas Amphioxus 



