March 22, 1888] 



NATURE 



499 



THE TEETH OF THE MYXINOID FISHES. 



T N the course of my work upon the morphology of the Verte- 

 brata, it has occurred to me to ascertain how far the generally 

 accepted account of the structure of the teeth in Cyclostomata 

 exhausts the facts at our disposal. The inquiry is one of extreme 

 interest in relation to the disputed affinities of this group with 

 the other fishes. It is well known that Balfour regarded the 

 Myxinoids as the survivors of a very primitive group which had 

 never possessed true jaws. Dohrn, on the other hand, while 

 holding that these fishes retain very many primitive characters, 

 has always asserted their degenerate nature as a canon of his 

 doctrine of the ancestry of Vertebrates. He has endeavoured to 

 produce evidence of this in several of his " Studien," but so far 

 as I am aware, the secondary character of the sucking mouth of 

 the group has never yet been fully proved. 



In Balfour's "Comparative Embryology" (vol. ii, p. 264), we 

 read, " I am acquainted with no evidence, embryological or 

 otherwise, that they (the Myxinoid fishes) are degraded gnatho- 

 stomatous forms." 



As the nature of the mouth in this group was one of Balfour's 

 arguments against Dohrn's gill-cleft origin of the mouth of all 

 Vertebrates, 1 and as my own views of the nature of the hypo- 

 physis cerebri are also affected by Balfour's reasoning, I may 

 perhaps be allowed to state why I attach great importance to 

 the structure of the teeth in the Myxinoids. With the exception 

 of these animals and Amphioxus, all Vertebrates are known to 

 possess true teeth anl true jaws ; but it appears to me that if 

 it can be shown that the Myxinoids present traces of true teeth, 

 it must be assumed that they once had true biting jaws. For 

 true teeth are necessary appendages of biting jaws, while they 

 are never found except when true jaws are present. It is 

 important to note that Huxley long ago insisted upon the 

 presence, in the lamprey, of a true mandibular jaw- apparatus, 

 homologous with that of the gnathostomata. 



All previous investigators of the group, from Johannes Miiller 

 to Parker, have described only the horny nature of the teeth, 

 and that simply because no one has till now made microscopical 

 sections of them. It must here suffice to point out that the 

 current view is correct only so far as the Petromyzontidae are 

 concerned. They alone possess only horny teeth. In Petro- 

 myzon marinus, these are curiously complicated, in th.nt they are 

 represented by three horny cusps or thimble-like bodies lying 

 one upon the other, and each arising in a special groove at the 

 base of the tooth. (Prof. Howes writes me that he has long 

 known of this fact.) 



Myxine and Bdellostoma, which retain many more primitive 

 characters than the Petromyzontidae, possess true teeth in the 

 sense of those of other Vertebrates. These are hidden by the 

 aforenamed horny cones, which are formed above them, and, in 

 fact, each horny tooth in these two genera has a true odonto- 

 blastic pulp underlying it. The following is a brief description 

 of the appearance of such a tooth in longitudinal section, as 

 exemplified in Bdellostoma. Outside all is the bright yellow 

 homy layer, formed from a "horn groove" at the base of the 

 tooth. Within this is a stratified epithelium, which extends 

 inwards as far as the true tooth ; I am unable, however, to find 

 any modified layer of epidermic cells which might represent the 

 so-called enamel organ of other developing teeth. The true 

 tooth is mainly composed of a very hard conical cellular mass, 

 which is probably calcified (I have not yet tested it chemic- 

 ally). It pos esses a true pulp-cavity with blood-vessels, &c., 

 while it is made up of cellular elements, which are arranged in a 

 somewhat radiate fashion. The cells are hard, possessed each 

 of a large nucleus longitudinally striated, especially at the apex 

 of the tooth and near the surface. 



The apex of the cone is surmounted by a small cap of bright 

 transparent structureless matter, which is either dentine or 

 enamel ; from its appearance, and from the fact that the pulp is 

 very hard and obviously calcified, I am inclined to regard it as 

 an enamel structure. While as yet it is not possible to follow 

 the development of this cap, it appears to me to be a secretion 



' Amphi jxus is here left entirely out of acount. Personally, I do not 

 intend to commit myself in seeking to compare any organs of Amphioxus 

 with those of the higher Vertebrates. I would rather leave Amphioxus 

 alone, but 1 may at least remark the possibility that the mouth in Amphi- 

 oxus may turn out to be the hom )logue of the hypophysis — gut pass.ige in 

 Myxine and Bdellostoma. The fact that no hypophysis has yet been dis- 

 covered in this animal is only in accordance with o.her negative comparisons 

 between it and other fishes. 



of the pulp-cells ; and, should it turn out to be enamel, we shalF 

 have striking confirmation of the enderonic origin of that layer, 

 advanced by Huxley more than thirty years ago. I, for one, 

 do not believe his view to be .so improbable as is generally 

 supposed. 



The teeth of Myxine present essentially the same structure as 

 those of Bdellostoma ; they are, however, smaller, weaker, and 

 more degenerate, for the cap of enamel (or dentine) is, in them, 

 reduced almost to nothing — indeed, it can only be found after very 

 careful search, and I think that from some of the teeth it is entirely 

 absent. 



With this discovery, true teeth come to be characteristic of all 

 the lowest Vertebrates except the outcast Amphioxus, and thus 

 the gulf separating the latter from the former becomes widened. 

 Some zoologists explain the absence of spinal ganglia in Am- 

 phioxus by assuming that they are still within the spinal cord : 

 might one hint that they can now also suppose that the teeth of 

 Amphioxus are still within the gums? 



In view of the facts here stated it becomes an interesting 

 question for the palaeontologist as to how far the " Conodonts " 

 really are the remains of Myxinoid teeth. Zittel's view that 

 they are really Annelidan teeth seems to me the more probable 

 one {Handbuch der Falaontologie, Bd. iii. p. 38). 



J. Beard. 



Anatomisches Institut, Freiburg i/B, 



MODELS ILLUSTRATING THE MODIFICA- 

 TION OF THE ARTERIAL ARCHES IN 

 VERTEBRATES. 



T_r AVING recently, with the help of my assistant, made some 

 simple and inexpensive models illustrating the modifica- 

 tions of the arterial arches in Vertebrates, which I find very 

 useful for purposes of demonstration, I send a short description 

 of them to Nature. Students, as a rule, find it difficult to 

 understand figures of these structures, and a model, in three 

 dimensions, gives a much more accurate idea of their general 

 relations than any drawing can do. 



My models are founded mainly on the figures given by Boas, 

 in his paper " Ueber die Arterienbogen der Wirbelthiere " 

 {Morphol. Jahrbuch, Band xiii. Heft i). 



The various vessels are represented by stout brass wires (about 

 g-inch in diameter), bent to the proper form and soldered to- 

 gether ; and each model is made, in the first place, to represent 

 six arches. In the case of the fish, the ventral aorta and lower 

 half of each arch (representing the afferent branchial trunk) is 

 painted blue, to indicate that the blood contained therein is 

 venous ; the upper half of each arch (representing the efferent 

 trunk), together with the epibranchials and dorsal aorta, are 

 coloured red, to show that they contain arterial blood. The 

 heart is modelled out of modellers' clay, and fixed on to the 

 ventral aorta before being dried ; it shows the typical parts of 

 the fish-heart, and is painted blue. 



At present I have only made two other models, representing these 

 structures in air-breathing Vertebrates, the types taken being the 

 frog and the mammal. In these, similar colouring is used, but 

 those parts which disappear in the adult are painted white. Tiie 

 various parts of the heart are also coloured red or blue, according 

 to the nature of the blood contained in them. 



Thus, in the frog the left auricle is red, the right auricle and 

 sinus venosus blue, and the ventricle purple, to show the mixed 

 character of the blood. The first, second, and fifth arches, the 

 portion of the epibranchial between the third and fourth arches, 

 and the ductus 13otalli of the sixth arch, are white ; the third arch 

 (carotid and lingual artery), red ; the fourth (aortic) arch and 

 dorsal aorta, purple ; and the lower part of the sixth (pulmonary), 

 blue. In the mammal, the left side of the heart, the left aortic 

 arch, dorsal aorta, and carotids, are red ; the right side of the 

 heart, and the pulmonary artery, blue ; and the remaining parts, 

 which disappear in the adult, white. 



The paint I have used is Aspinall's oxidized enamel. 



As this method of illustrating blood-vessels is also particularly 

 useful for lecture-pui poses, I intend, later on, to model whole- 

 vascular systems in the same way, 



W. N. Parker. 



University College, Cardiff. 



