value in a study of placentation. This is a kind of special- 

 ization that, by its appearance in simpler forms, suggests 

 that it is not particularly, important in revealing basic 

 phylogenetic relationships. It is, however, certainly an inter- 

 esting topic for functional investigation. 



LARVAE 



The larvae of vertebrates are of interest since they may 

 indicate the primitive body form of this group (Figures 

 7-21 to 7-23). They may only indicate a developmental 

 specialization brought about by the increase in size of the 

 vertebrate. There is no evidence that the larval form is like 

 the primitive or original stage on which the new adult form 

 has been superimposed. All evidence seems to suggest that 

 the larva, although it may be simpler in its structure, can 

 be as highly specialized as the adult. The direct develop- 

 ment observed in the higher forms is probably also a modi- 

 fication. 



The larvae of vertebrates share various features in com- 

 mon. The general body organization includes a head, trunk, 

 and tail, and there are two pairs of limbs. Development of 

 these limbs appears to be direct to the fin or the foot. 

 Larvae often have both external and internal gills. The ex- 

 ternal are perhaps a specialization for small size and sim- 

 plicity, and the internal represent the adult type. 



The larvae agree in that they undergo some sort of meta- 

 morphosis. This is well marked in the amphibian since it 

 involves a conversion from an aquatic adaptation to an air- 

 breathing terrestrial habit. However, a metamorphosis oc- 

 curs in Amphioxus and the lamprey, which remain aquatic. 

 Metamorphosis thus appears to be a basic part of the devel- 

 opmental sequence. This basic feature involves specializa- 

 tion of the larva in one direction, the adult in another. 

 Metamorphosis may involve transfer from microscopic to 

 macroscopic food. 



external gi 



A PROTOPTERUS 



anterior nasal 

 opening 



adhesive organ or sucker 



NEOCERATODUS 



Figure 7-22. External form of early larvae of Protopterus, A, 

 Lepidosiren, B, and Neocerotodus, C. (After Budgett, Kerr, Semon) 



f#iggp^P®*« 



anus 



oral sucker 

 mouth RANA PtPIENS 



nostril 



external gills 



fore limb 



balancer 



AMBYSTOMA MACULATUM 



B 



nostril 



fore limb 



POLYPTERUS 



B LATE LARVA 



Figure 7-21. External form of early larva of Polypterus, A, and the 

 form of the external gill of a later larva. (After Kerr, 1907, and Stein- 

 dachner, 1869) 



NECTURUS MACULOSUS V- 



Figure 7-23. External form of larvae of Rona pipiens. A, Ambysfomo 

 mocu/atum, B, and Necfurus maculosus, C. (A and B after Witschi, 1 956; 

 C after Nelsen, 1953) 



214 • EMBRYOGENESIS OF THE CHORDATES 



