XXVIII GILLS 357 



the rectum is produced into a longitudinal ridge, or typhlosole 

 {ty), and two similar ridges begin in the stomach and are 

 continued into the first portion of the intestine. The 

 stomach contains at certain seasons of the year a gelatinous 

 rod, the crystalline style. 



The gills consist, as we have seen, of two plate-like bodies 

 on each side between the visceral mass and the mantle : we 

 have thus a right and a left outer (Fig. 93, B, O. G), and a 

 right and a left inner gill (/. G). Seen from the surface 

 (Fig. 94), each gill presents a delicate double striation, 

 being marked by faint lines running parallel with, and by 

 more pronounced lines running at right angles tq, the long 

 axis of the organ. Moreover, each gill is double, being 

 formed of two similar plates, the inner and outer lamella, 

 united with one another along the anterior, ventral, and 

 posterior edges of the gill, but free dorsally. The gill has 

 thus the form of a long and extremely shallow bag open above 

 (Figs. 94 and 95) : its cavity is subdivided by vertical plates 

 of tissue, the inter-lamellar junctions (Fig. 95, /'. /. j), which 

 extend between the two lamellae and divide the intervening 

 space into distinct compartments or water-tubes {w. t), 

 closed ventrally, but freely open along the dorsal edge of 

 the gill. The vertical striation of the gill is due to the fact 

 that each lamella is made up of a number of close-set gill- 

 f laments (/) : the longitudinal striation to the circumstance 

 that these filaments are connected by horizontal bars, the 

 inter-filatnentar junctions (i. f J). At the thin free or ventral 

 edge of the gill the filaments of the two lamellae are con- 

 tinuous with one another, so that each gill has actually a 

 single set of V-shaped filaments, the outer limbs of which 

 go to form the outer lamella, their inner limbs the inner 

 lamella. Between the filaments, and bounded above and 

 below by the inter-filamentar junctions are minute apertures. 



