298 PROFESSOR FRANK J. COLE 



D. The Respiratory Organs (Figs. 2, 6, and 12). 



The coarse anatomy of the breathing apparatus of the Myxinoids has been so often 

 described that an acquaintance with it may be taken for granted. Further, the general 

 structure is sufficiently indicated in the reconstruction given as fig. 2. The peri- 

 branchial or pleural sacs, and the relations of the gills to them, are fully described in 

 my Part IV. pp. 220-222. I may, therefore, at once proceed to a more detailed 

 account of the anatomy of the parts. 



1. Structure of the Gills. 



Under the serous membrane with its long flat nuclei the outer gill wall exhibits a 

 loose connective-tissue layer in which muscle fibres are lodged. They are in two sparse 

 layers, an external circular or concentric, the axes being the gill ducts, and an internal 

 radial. The concentric fibres do not pass right round the gill, but each bundle of fibres 

 forms only a more or less short segment of the circle. The fibres histologically are 

 intermediate between the plasmic and aplasmic types described in my second part. 

 They are distinctly striated ; the nuclei are large, vesicular, and peripheral ; but there is 

 only a slight quantity of sarcosplasm present, most of it being collected round the 

 nucleus. The fibres are somewhat flattened, and average about 37 "5 m by 12 m, and 

 both the transverse and longitudinal striation show up with diagrammatic clearness 

 in material stained with iron hsematoxylin. 



The fundamental ground plan of the Myxinoid gill is quite simple. There is a 

 tube connecting the gut with the exterior, which we may call the gill duct. At one 

 place the epithelial lining of this duct throws out a number of lamelliform evaginations 

 — about 8-10, but the number in the adult is difficult to enumerate owing to secondary 

 modifications — the long axes of which are radial and the short axes parallel to the long 

 axis of the duct. Hence at this place a considerable swelling of the duct supervenes 

 to form the cake- or pouch-like gill. The unmodified proximal and distal portions of 

 the duct remain as the afferent and efferent gill ducts respectively (figs. 2, 6, and 12, 

 a.g.d., e.g.d.) 



It therefore results in a section at right angles to the long axis of the duct itself 

 that we have a star-shaped arrangement. There is a central cavity, the cavity of the 

 duct, and a number of compressed radial chambers opening into it. The tissue between 

 the walls of contiguous evaginations is highly vascular, and constitutes the respiratory 

 apparatus of the gill. This tissue projects inwards from the outer wall or periphery 

 of the gill as a number of radial plates, each one clothed by two sheets of epithelium — 

 the walls of two adjacent evaginations. A gill lamella, therefore, is formed by the 

 contiguous walls of two epithelial evaginations plus the intervening vascular tissue. 



The afferent gill artery (afhr.), on reaching the gill, forms a circle round the 

 efferent gill-duct (cp. fig. 6). From this circle a number of radial arteries arise like the 



