STRUCTURE OF THE VERTEBRATES 33 



are sufficiently like those of Amphioxiis to be understood with- 

 out repetition. The embryo hatches as a larva which shows rela- 

 tionship with the primitive chordates. 



The larva is strikingly like Amphioxus in body shape and 

 in structure, though (1) the reduced number of gill slits, (2) 

 the more advanced brain, and (3) the chondrocranium are 

 typically vertebrate. Even in the early larva the ventral hepatic 

 outpocketing, which remains in Amphioxus as a caecum, has 

 become a glandular liver. The similarities are in the oral hood, 

 the myotomes, and the pharyngeal structures. The food getting 

 structures of the pharynx are a ventral endostyle and a dorsal 

 epipharyngeal groove. Both are ciliated and secrete mucus. 

 After three or four years the larva metamorphoses into the adult 

 tj^pe. The dorsal part of the pharynx, including the upper 

 groove, pinches off as an esophagus, leaving the pharynx as a 

 blind sac. The endostyle also closes, becomes at first a blind 

 pouch with a slight pharyngeal connection, and then the con- 

 nection is lost as the pocket becomes glandular. The endostyle 

 is then the thyroid gland. This is one of the clearest homologies 

 in vertebrate anatomy, the thyroid of higher types always aris- 

 ing as a median, ventral outpocketing of the pharj-nx. 



Specializations of Cyclostomes. The larva of the lamprey 

 shows such definite relationships with Amphioxus, and the fossil 

 forms are fundamentally so similar to the living representatives, 

 that most anatomists consider the class as very primitive, 

 although highly specialized, vertebrates. Other anatomists con- 

 sider them as completely degenerate vertebrates. The evident 

 degeneracy of the myxinoids as regards the branchial basket, 

 the eyes, and the fins is used as evidence for the latter theory. 

 The research on the ostracoderms, on the other hand, gives 

 weight to the older theory. 



The fact that all known ostracoderms had some form of 

 skin, or dermal, bone indicates that these structures may have 

 once existed in all cyclostomes and have been lost in the living 

 orders. According to this theory the ostracoderms lost their 

 free-swimming ability with the development of armor and be- 

 came extinct. 



