D.— ZOOLOGY. 



93 



formation of this so-called ' marginal slit,' which of course is also manifested 

 by a corresponding gap in the shell. As growth proceeds the viscera 

 behind occupy an increasing proportion of the space below the shell, and 

 the gill-chamber itself shifts forwards, so that continual readjustment of 

 slit to cavity is required. In Emarginula the mantle maintains the slit 

 at its edge throughout life. As the mantle extends, the slit extends ; but 

 the intact part of the mantle behind also extends, and seals up with a 

 secondary deposit the older parts of the slit in the shell. Thus arises a long 

 seam in the shell, the so-called ' slit-band,' which marks the track along 

 which the slit has travelled. In Fissurella, the Key-hole Limpet, a 

 different arrangement prevails : as soon as a slit of sufficient size has been 



Fia. 6. — Post-larval Development of Fissurella. 



produced, the edges of the mantle meet in front of it and fuse, the'mantle 

 thus regaining its original integrity. The shell now goes on growing as 

 intact as a Limpet's, leaving a hole near the summit which retains com- 

 munication with the gill-cavity throughout life, and is enlarged from time 

 to time by absorption. In Haliotis the mantle goes on splitting and 

 closing throughout life, thus adding to the number of holes instead of 

 enlarging the first one. The first and its successors are sealed up, one 

 after the other, as the mantle-chamber grows forwards into new 

 positions. 



These various arrangements of slit and pores, with numerous inter- 

 mediate conditions, are adaptive to minor differences of body-form and 

 habits. In some the gills are equal and symmetrical, in others unequal 

 and squeezed to one side ; the body may be tall and pyramidal, or flat 

 and broad, the shell accordingly conical or spiral, and carried above the 

 foot or brought down to the substratum. In all, whether symmetrical 

 or asymmetrical, the slit or series of holes lies in the morphological median 

 line, between the two gills, and is associated with a persistence of the 

 original bilateral arrangement of the inhalant respiratory currents. To 

 its presence beyond all doubt the Zygobranchia owe the preservation of 

 their original pair of gills in the reversed mantle-chamber. As soon as the 

 right gill goes (Azygobranchia) the slit goes too, and a new current, 

 oblique in direction, but simple instead of complex, is set up through the 

 chamber, water entering in front on the left side, bathing the persistent 

 (left) gill, then crossing to the right side to which the anus is diverted, 

 alongside the persistent (right) kidney. Entrance and exit are each 

 defined by special folds of the mantle-edge, which may be drawn out into 



