302 PROFESSOR FRANK J. COLE 



free border. Occasional mitoses are seen, but there are no gland or mucous cells. A 

 few of the glassy mucous cells were, however, seen in the afferent gill duct, and Schreiner 

 says that there are "by far not as many slime cells in the afferent gill ducts as in the 

 mouth." The muscle fibres are, on the whole, similar to those of the gills, except that 

 they are coarser, and the striation is not so distinct. 



The external branchial apertures are conspicuous openings situated far back near 

 the middle line on the ventral surface, just in front of the anterior extremity of the 

 pre-anal " fin," and a little behind the anterior third of the body. They are always 

 asymmetrical as regards size, but not invariably so as regards position. The right is 

 always the smaller of the two (and naturally so, as the ductus oesophago-cutaneus only 

 occurs on the left side), and it is situated a little in front of the left ; externally the left 

 opening is sometimes partially, but rarely completely, divided into two — in the latter 

 case the ductus oesophago-cutaneus having a separate external aperture. In 1815 Sir 

 EvERARD Home published a figure by William Clift in which three such openings are 

 indicated. J. Muller, however, states that he has never seen this, but Maas finds in 

 his youngest specimens the branchial cloaca and the ductus oesophago-cutaneus entirely 

 separate, so that only their openings on to the outer skin come together, like a pair of 

 spectacles. He supposes therefore that the common duct is only formed with age, bur 

 adds that the ductus never opens medially, but always on the left. There is no doubt 

 in fact that the ductus is a potential left gill. When the ductus opens separately the 

 two apertures are not side by side, as in Clift's figure, but antero-posterior, the smaller 

 anterior one being the orifice of the branchial cloaca of its side, and the larger posterior 

 one that of the ductus. Internally, of course, the two structures are always distinct. 



The structure of the gill of Bdellostoma has been described by Jackson (30). He 

 has evidently, however, directed but little attention to this section of his work, and his 

 scheme of the circulation, representing, as it does, one side of the gill as arterial and the 

 other as venous, is manifestly unsound even on a priori grounds. He has failed here to 

 recognise the fundamental fact of the interdigitation of the afferent and efferent vessels 

 — a condition absolutely essential if the gill is to be a respiratory organ at all. Again, 

 his fig. VIII. is a misinterpretation of the apparent dendritic structure of the gill 

 lamella as seen in vertical section. As I have pointed out above, it is fatally easy to 

 commit this error, especially if one does not realise how impossible vascular culs-de-suc 

 must be in the Chordate gill. 



The development of the Myxinoid gill follows the course we should deduce from a 

 knowledge of its anatomy. In Bdellostoma, according to Stockard (52), the gill is 

 originally tubular, only the oesophago-cutaneus duct remaining in the tubular stage 

 throughout life. The gill tubes are entirely endodermal. At the point where the future 

 gill pouch is to develop the lumen of the tube becomes enlarged, and then, by a folding 

 of the walls, the characteristic radiate gill is produced as outlined in the anatomical 

 description above. 



