bones and a part of it is indicated by grooves. The sensory 

 lines of primitive amphibia generally lay in the skin over 

 the bones, but grooved the bones in some areas. 



The pattern of the sensory lines in the ichthyostegid, or 

 other early amphibians, is like that of the osteolepid crossop- 

 terygian. No occipital canal is evident, although it is 

 usually indicated in illustrations oi Ichlhyostega; in later am- 

 phibians a groove for this canal is sometimes observed on 

 the posterior margins of the postparietals and tabulars. In 

 some amphibians a loop is observed on the squamosal and 

 quadratojugal (Figure 4-24), indicating perhaps that the 

 jugal extension leads into the proximal end of the oral 

 canal. Modification of the canals in the early Amphibia, as 

 indicated by their looping, makes interpretation difficult. 



In living amphibians, the more aquatic salamanders 

 (Nectiirus and Cryplobranchus) have many neuromasts on the 

 surface of the skin. These neuromasts, arranged as lines, 

 are raised on elevations and are not pigmented. The lines 

 can be identified as the supraorbital, infraorbital, mandib- 

 ular and hyoid arch. There is a jugal extension connecting 

 the last tvk-o. The temporal section is undeveloped; there is 

 a middle pit line and an occipital connective. On the body 

 are dorsal, lateral, and ventral lines. These lines diffuse in 

 some areas, suggesting multiplication and spreading out of 

 the neuromasts over the skin. In the larvae of anurans 

 (Rana) these same lines are observed. 



A sensory-line system is lacking in reptiles, even in those 

 of aquatic adaptation. However, sensory organs, the proto- 

 trichs, are present and have much the same distribution on 

 the body as those of adult anurans. Whether these were 

 derived from lateral-line organs is not known. 



General observations On the basis of the lateral-line sys- 

 tem, the amphibian is like the crossopterygian or dipnoan 

 in that the infraorbital and hyoid lines meet below the eye. 

 The equal development of mandibular and hyoid arch lines 

 in aquatic salamanders may be explained as due to reten- 

 tion of the mandibular as surface organs, while the hyoid 

 was reduced from a canal to similar surface organs. 



Although several elaborate schemes have been suggested to 

 account for the various patterns of lines, it is best to apply 

 the law of parsimony and use the simplest possible explan- 

 ation. The basic pattern suggested in Figure 13-25 B ap- 

 pears to meet all tests; however, it is hypothetical. Again it 

 seems that the agnath agrees no better with the hypothe- 

 tical ancestral type than the gnathostome; that is, it is 

 modified and not ancestral. 



Other acustico-lateralis organs 



Ampullae of Lorenzini are observed in clusters on the 

 heads of sharks (vesicles of Savi in Torpedo), holocephalans, 

 sturgeons, and one marine teleost, Plotozus anguillans, a silu- 

 roid. These organs are mucous-filled tubes opening at one 

 end through the skin. Sensory and secretory cells lining the 



tube are innervated by the seventh nerve. The blind end of 

 the tube may be subdivided into several ampullae. These 

 organs are thermosensory and are perhaps also pressure 

 receptors. 



The rostral sac of Latimeria can be described here al- 

 though it may not be related to the ampullae of Lorenzini. 

 The rostral sac, or internasal organ, is a large cavity in the 

 snout which opens anteriorly through paired tubes and pos- 

 terolaterally by two pairs of tubes (Figure 4-33). The cavity 

 is largely filled by a vascular (venous) mass which is attached 

 dorso and ventroposteriorly; its va.scular connections are 

 ventral. The lining of the sac is a mucous membrane except 

 at the openings into the tubes where it is irregularly thickened 

 and pitted. These pitted areas are richly supplied by fibers 

 of the ophthalmic profundus nerve (Vi). The lumen of the 

 sac and its tubes are filled with mucous. The role of this organ 

 is unknown but it may be a thermoreceptor. 



GENERAL OBSERVATIONS ON THE 

 NERVOUS SYSTEM 



Here, as in other areas of their anatomy, the vertebrates 

 show a remarkable homogeneity in general plan and even 

 in many details. The tetrapods differ one from the other 

 only in minor details, and there is scarcely a structural gap 

 between them and the fishes. The agnaths are most dis- 

 tinct but the differences are seldom clear or really impres- 

 sive. The differences between the two living agnaths are 

 nearly, or actually, as great as those between agnath and 

 gnathostome. 



Several points develop from a consideration of the differ- 

 ences. Since the eye is degenerate in living cyclostomes, 

 would it follow that the ear is also retrogressive, i.e. that 

 three canals preceded two? There is no evidence to support 

 the conclusion that an inner ear with two canals gave rise 

 to one with three. Also it is unlikely that the myxinid type, 

 with one canal, is the most primitive; it is more probable 

 that it is a regression from the two-canal condition. Thus 

 the conclusion is reached that the otic vesicles underwent 

 change, involving canal formation, in two separate lines of 

 vertebrates, agnath and gnathostome. 



Paired nasal capsules, as observed in the gnathostome, 

 appear to be primitive, while the fusion of these in the 

 agnath is probably a modified condition. The sensory-canal 

 pattern of the agnath could be more primitive, but any at- 

 tempt to superimpose it on the gnathostome results in dif- 

 ficulties. Parallelisms are more strongly suggested here than 

 elsewhere; this involves the derivation of agnath and gnatho- 

 stome patterns from that of a common ancestor, quite distinct 

 from either. As a last point, if the common ancestor had a 

 trigeminal nerve with three main divisions, would not this 

 suggest that the jawed condition, or a situation leading to 

 the development of jaws, preceded that of the agnaths as we 

 know them? 



It would appear that the common ancestor of agnath and 



418 • THE NERVOUS SYSTEM 



