198 THE RESPIEATORY SYSTEM [CH. 



distally. The tracheae and blood-spaces are enclosed in a mass 

 of reticulated tissue, which divides the interior of the gill into 

 a large number of separate alveoli. This internal arrangement of 

 parts is more fully explained in the account given for the saccus (see 

 below), the histology of the two forms of gill being closely similar. 



The saccus (fig. 86 A, B) is covered by an unpigmented cuticle 

 (cu), which usually carries small hairs at intervals. Beneath the 

 cuticle is a continuous layer of polygonal hypoderm cells (hy) with 

 large oval darkly staining nuclei. These cells are filled with a 

 purplish-brown pigment. The boundaries between the cells are 

 pale. At certain points the hypoderm-layer is turned inwards (B), 

 so as to form a kind of delicate internal lamina, which probably 

 helps to support the very flimsy internal structure of the gill. 

 From the hypodermis of these laminae, as well as, apparently, 

 from a number of points on the main periphery of the hypodermis, 

 delicate strands of tissue pass inwards to form an immense number 

 of more or less spheroidal chambers or alveoli (al) which almost 

 completely fill up the internal cavity of the gill. Each alveolus 

 has, at some point on its wall, a single large nucleus (nu) not 

 unlike the nucleus of a hypoderm cell. Most of the alveoli appear 

 to be quite empty, but some clearly contain blood-plasma. 



Near the base of the saccus there are usually four main 

 longitudinal tracheae (tr), two of which remain symmetrically 

 placed and unbranched for some distance. On either side of 

 these lies a very large prolongation of the haemocoele, in which 

 both blood-plasma and blood-corpuscles can be clearly seen. One 

 of these large blood-channels is placed dorsally, the other ventrally, 

 as in the triquetral gill. The two channels become confluent 

 distally. In the median gill, the dorsal channel is afferent, the 

 ventral efferent ; in the laterals, the circulation is reversed. The 

 other two tracheae branch out into the cavity of the gill, the 

 branches (tr r ) travelling obliquely towards tke gill-border. Further 

 along the gill, all four tracheae give off branches. These branches 

 in their turn divide, until there are seen very numerous minute 

 tracheae at all points of the periphery. These, on reaching the 

 hypodermis, turn so as to run parallel to and close alongside it, 

 and in some places appear to press in so as to be almost contiguous 

 with the cuticle. 



