135 



however, lias found them close to the efferent vessels in 

 several of the Bracyhura. They do not appear to be 

 present in Cancer. 



In longitudinal sections through the gill (fig. 66) 

 the lamellae are seen to be lined by epidermal cells 

 (<-/i. ep.]. There is a narrow cavity containing blood 

 separating the upper and lower layer of cells. This 

 cavity, or lamellar sinus (l.s.) is bridged over in certain 

 parts by the junction of the two layers of epidermal cells. 

 At the free edge of each lamella the lamellar sinus is 

 continuous with the larger outer lamellar sinus (o. I. s.}. 

 This runs around the edge of the lamella, and the 

 epidermal cells in this region are extremely flattened. 

 Each lamellar sinus is in contact with the afferent 

 branchial vessel on the outer side, and the efferent 

 branchial vessel on the inner side. It is in the lamellae 

 that the aeration of the blood is effected. 



Dorso- ventral muscles (PI. VIII, fig. 51, 

 PL IX, fig. 56, d. v. m.). 



Extending upward from the membranous roof of the 

 branchial chamber to the carapace, is a series of muscles 

 which may be termed the dorso-venti-al muscles. The 

 arrangement of these muscles will not be described in 

 detail. There are three sets of muscles at each side (see 

 fig. 51). The outer and middle series are arranged in two 

 parallel lines running antero-posteriorly. The inner set 

 is small, and is situated above the inner region of the 

 branchial chamber. The roof of the chamber is consider- 

 ably lower at the anterior end than in the posterior region, 

 so that the anterior muscles are consequently longer than 

 the posterior muscles. 



Since the roof of the branchial chamber is soft and 

 membranous, it is capable of considerable movement. The 



