46 



the bipartite remnant of the bipartite eleventh sternite in the 

 case of the two larval gills. The muscles which control the 

 movements of the gills are small but strong, and are inserted 

 into the walls of these basal pieces. The nature of the joint 

 between the basal piece and the gill is such that, if the gill be 

 seized by an enemy, it can be cast off without injury to the basal 

 piece. Such a joint is known as a hreakinc/-joint(8, 18). 



Morphologically, each caudal gill is a hollow outgrowth of the 

 body-wall, lined by a tough, chitinous integument or cuticle, 

 beneath which lies a continuous layer of hypoderm-ceUs. The 

 space enclosed by the gill- walls always carries one or more large 

 longitudhiol frachece, from which more or less numerous branch- 

 trachea^ pass outwards to all points of the gill-wall. Within the 

 gill, also, there are always developed two hlood-canals, or pro- 

 longations of the hnemocoele. One of these is placed dorsally, 

 the other ventrally. These two canals become confluent distally 

 within the gill, so that there is a continuous circulation of blood, 

 corpuscles, passing into the gill and out of it again. The trachea^ 

 and blood-canals are supported in a more or less strongly devel- 

 oped meshwork of alveolar tissue, which fills up all the rest of 

 the space in the interior of the gill. 



The Hyjwdermis and Cuticle. 



The wall of the gill, like that of the bod}^ of the larva, is 

 formed of a tough, chitinous cuticle {cu, cu), beneath which lies 

 a single layer of polygonal cells, the hypodennis. 



The cuticle varies in thickness from 5//, to as much as 15/x, 

 according to the type of gill. It is thinnest and most delicate 

 in 8accoid Gills, stoutest and strongest in Lamellar Gills. Along 

 the mid-ribs of the Triquetro-quadrate and Lamellar Gill-systems 

 it is especially thickened, and is in such places often armed with 

 stout spines. 



The cuticle can always be separated into two distinct parts, an 

 inner, softer part {cu), apparently formed of a number of thin 

 parallel layers, and not stainable with eosin, and an outer harder 

 part {cu!) of quite uniform structure, easily stainable to a deep 

 red with eosin. Of the total thickness of the cuticle, the outer 



