182 EMBEYOLOGY OF THE LOWER VEETEBEATES oh. 



CO, 



Fio. 102. — Larva of Sarcodaces odbe. (After Budgett, 1901.) 

 c.o, cement-organ. 



of endodermal pockets, they obviously recall gill pouches on the 



one hand and coelenteric pouches on the other. Their position 



suggests a pair of premandibular gill pouches : their function, that 



of forming an excretion (cement), perhaps indicates rather coelomic 



affinities and the present writer suggested (1906) their possible 



correspondence with premandibular head cavities of other Vertebrates. 



Eeighard and Phelps 

 (1908) homologize 

 them with the anterior 

 pair of head-cavities of 

 Elasmobranchs while 

 van Wijhe (1914) sup- 

 ports a homology with 

 the ciliated organ of 

 Amphioxus. 

 Altogether these cement-organs are very interesting and puzzling 



structures and would well repay further investigation. A thorough 



comparative study should be made of their development in the 



archaic Crossopterygians and of their possible homologues in 



Elasmobranchs. 



Little is known regarding cement-organs in Teleosts, though it 



is probable they will be found to occur in| various tropical fresh- 

 water fishes. Budgett 



(1901) found a large 



cement - organ on the 



head of the larva of the 



Characinid Sarcodaces 



odoe (Eig. 102, c.o). In a 



larva believed to be that 



of the Mormyrid Hyper o- 



pisus bebe he found six 



well-marked cement 



glands on the head which 



in this case secrete fine 



threads by which the 



larva hangs suspended 



in the water until the 



yolk is used up (Eig. 



103). Heterotis and 



Gymnarchus also possess similar organs — very small in the latter case 



(Assheton, 1907). 



The organs in these various fishes present the appearance , of 



being ectodermal thickenings : we have as yet no information as to 



whether, as may be suspected, they really originate from the 



endoderm. 



Digestive Teact. — The respiratory region of the alimentary 



canal is succeeded by the true digestive tract and this shows 



more or less pronounced differentiation into successive portions — 



Fig. 103. — Teleostean larvae, supposed to be those of 

 Hyperopisus bebe, suspended from the rootlets in the 

 nest. (From Budgett, 1901.) 



