62 THE BOOK OF NATURE STUDY 



which are the remains of the clefts originally existing uninter- 

 ruptedly between the separate filaments. Thus the whole gill- 

 plate of the adult is a closely knit trellis. If examined under the 

 microscope the entire surface of the gill is seen to be covered 

 with thousands of minute and vibrating hair-like processes, 

 termed cilia ; these also clothe the inner surface of the mantle, 

 " foot/* labial palps, and indeed the entire body. The action 

 of the cilia is to waft currents of water in at the inhalant siphon 

 forwards into the space ventral to the gill-plates, through the 

 lattice-work of the gills into the gill-cavity, then dorsalwards 

 into the space above the gills, and lastly backwards and out 

 of the shell at the exhalant siphon. The filaments of the gills 

 are hollow structures, and contain blood which is supplied with 

 oxygen and gives out carbon dioxide gas through the thin, 

 ciliated walls of the filaments as it courses through them. Re- 

 spiration is also carried on through the surface of the mantle- 

 folds, and probably in some degree through the skin of the " foot." 

 The orange-yellow colour which pervades the whole animal is 

 due to a pigment analogous to the haemoglobin of our own 

 blood, which has a marked affinity for oxygen. 



The attachments of the gill-sheets should be noticed, for they 

 effect the separation of the inward and the outward current of 

 water. The outer sheet of the external gill is united along the 

 entire length of its dorsal edge to the rriantle-fold ; its inner sheet 

 is similarly united to the dorsal edge of the outer sheet of the 

 internal gill. The inner sheet of the internal gill is attached 

 in its anterior region to the visceral portion of the " foot " ; in 

 its mid region it is free from all connections, and may be reflected 

 so as to expose the dark purplish-brown surface of the excretory 

 organ (nephridium) ; but in the posterior region the dorsal border 

 of this sheet is fused with its fellow of the opposite side. Thus 

 a transverse section through the gills, taken behind the " foot," 

 has the appearance of two W's joined together (WW), the two 

 outside strokes being joined at their summits to the right and 

 left mantle-lobes respectively. The inflowing currents travel 

 ventral to (below) the various parts of the W's ; the outflowing 

 dorsal to (above) them. A blunt probe, e.g. a knitting needle 

 or wax taper, can be passed backwards along the exhalant pas- 



