144 



THE BIOLOGY OF MARINE ANIMALS 



tion) by 63-64% in Spirographis spallanzanii and Bispira volutacornis. 

 Because the crown is a muscular and strongly ciliated organ its metabolic 

 rate is high, estimated at 26% of the total oxygen consumption of the 

 intact worm. These results indicate that about 38% of the respiratory 

 needs of the body is satisfied by gaseous exchange through the gills. In 

 some species respiration continues when the worm is withdrawn into its 

 tube, as the result of regular irrigation movements (56, 159, 160). 



Ciliated tentacles on the lophophore of ectoproct polyzoans and phoro- 

 nids are functionally similar to those of filter-feeding polychaetes. In both 

 groups they probably subserve respiration as well as feeding. Filter-feeding 

 gills are also found in lamellibranchs and brachiopods (Figs. 4.4, 5.8). In 



Supra.bra.nchla.1 

 cav " 



Fig. 4.4. Diagrammatic Section through a Lamellibranch 



to Show Arrangement of Gills 



(After von Buddenbrock, 1928) 



these animals the gills are enclosed within a shell, and a feeding and 

 respiratory current is circulated through the mantle cavity. The gills of 

 lamellibranchs are vascularized and participate in the respiratory ex- 

 change, but they are probably of secondary importance compared with the 

 general body surface and mantle lining. The enclosed lophophore of 

 brachiopods likewise maintains a current of water through the mantle 

 cavity. The vascular system is little developed in this group, but coelomic 

 spaces extend into the mantle and tentacles. Respiration is probably a 

 generalized function of the entire exposed epidermal surface (Chapter 5). 

 Ascidians and cephalochordates possess a branchial chamber, the walls 

 of which bear gill bars and gill apertures. The gill surface is very extensive, 

 and is primarily a filter-feeding mechanism. The ciliary currents, in addi- 

 tion, provide for respiratory exchange which occurs, in part, across the 

 gill membranes, and to an even greater degree over the entire integument. 

 Vascular systems, differently organized in sea squirts and lancelets, carry 

 dissolved gases to and from the sill filaments. 



