178 



VERTEBRATA 



Classification of CRANIATA. J 

 Grade A. CYCLOSTOMA. 



Class I. Myxinoidea. 



II. Pctromyzontia. 

 Grade B. GNATHOSTOMA. 



Grade a. Branchiata heterodactyla. 

 Class I. Pisces, 

 II. Dipnoi. 

 Grade b. Branchiata pentadactyla. 



Class. Amphibia. 



Grade c. Pentadactyla lipobranchia. 

 Branch a. Monocondyla. Branch, b. Amphicondyla. 



Class I. Rcptilia. Class. Mamtnalia. 



II. Aves. 



Ances- If we now briefly consider what must have been the common 

 tral form ancestral form from which these Oraniata have proceeded, making 

 ofCVani-use of such internal evidence as their structure affords, we find 

 ate. that we get no further back than such an animal as would fit the 

 description given above, with the exception that we should be 

 warranted in substituting in the ancestor a pair of continuous 

 lateral fins, with comb-like cartilaginous skeleton, in place of the 

 two pairs of fins, or their total defect, seen in living Craniata. We 

 get no clear suggestion from the study of Craniata themselves as 

 to the meaning of the curious shape of the brain and its outgrowths 

 (though the pineal outgrowth has recently been explained as an 

 eye), nor as to the original genesis of the notochord. We should, 

 however, be justified in representing that region which now cor- 

 responds to the hinder part of the skull and brain as more fully 

 developed and segmented, so as to give a series of separate myotomes 

 and perhaps separate nerves corresponding to the several furcal 

 branches of the vagus ; and we may very well suppose that the num- 

 ber of pharyngeal gill-slits was larger in the ancestral than in any 

 living form, though it seems improbable that in any true Craniate 

 did each gill-slit correspond to a distinct muscular segment. 



An attempt to go further than this has been made by Dr Anton 

 Dohrn by the method of hypothesis and subsequent corroborative 

 inquiry into facts of minute structure and embryological history. 

 Making use of the principle of degeneration, Dohrn started with 

 the legitimate hypothesis that the branches of Vertebrata other 

 than Craniata viz., Cephalochorda, Urochorda, and (though at the 

 time he commenced his work their structure was not fully under- 

 stood) Hemichorda were not to be regarded as permanent records 

 of steps in the evolution of Craniata, but rather as greatly de- 

 generate offshoots from the ancestors of that group, which could 

 throw but little light on the character of their non - degenerate 

 ancestors. A second fundamental assumption which led Dohrn to 

 his position was that the segmentation of the Craniates' body-wall 

 is a primitive and essential feature in their structure, and becomes 

 more and more fully expressed instead of less developed the further 

 we go back in their ancestry. Dohrn, in fact, assumes that what 

 is called metameric segmentation is a phenomenon of structure 

 which has occurred once only in the history of animal form, and 

 that all segmented animals are genetically related and descended 

 from a common segmented ancestor. Assuming this, he pointed 

 to the existing Chsetopod Worms as most nearly representing at 

 the present day the common ancestor of segmented animals. They 

 have, as he pointed out, a high organization, little inferior to that 

 of the lowest Craniata; they possess a well developed ccelom, 

 blood-vessels with red blood, a segmental series of nephridia 

 (modified in some as gonaducts), segmental branchiae, and lateral 

 locomotive organs ; not a few develop cartilage as a skeletal sup- 

 port ; and many show a concentration and fusion of segments to 

 form a complex head, which resembles, so far, that of Craniata. 

 The ventral in place of the dorsal position of the nerve-cord led 

 Dohrn to accept De Blainville's conception that the dorsal and 

 ventral surfaces are reversed in Vertebrata, as compared with 

 Annelids, Crustaceans, and Insects, so that the Vertebrate is com- 

 pared to an Insect walking with its ventral surface upward. This 

 led further to the notion that the mouth of the Ch&topod or 

 Annelid, which penetrates the nerve-cord, or rather passes between 

 its two divaricated lateral constituents in those animals, has in 

 Craniata disappeared, its place being taken by a new mouth de- 

 rived from the modification of a pair of gill-slits. The remnant of 

 the old mouth, which should, if the comparison instituted holds 

 good, lie in Craniata somewhere on the dorsal surface of the cranial 

 region, was sought by Dohrn in some of the peculiar and hitherto 

 unexplained median structures of the brain : at one time the fourth 

 ventricle with its deficient roof was suggested as thus to be explained, 

 whilst subsequently the curious median structures, the pineal and 

 pituitary bodies, were called in as possibly thus significant. 



Without pursuing further the elaboration of Dohrn's views, it 

 must be at once noted that, whilst the legitimacy of the assumption 

 of degeneration must be admitted, the second assumption, viz., 

 that metameric segmentation is a character bringing all forms 

 showing it into a special genetic continuity, cannot be accepted. 

 1 The classes here enumerated are described in separate articles, whilst 

 Cydostoma and Dipnoi are included in the article ICHTHYOLOGY. 



The property of repeating units of structure, so as to build up a 

 complex of many similar parts united to form one individual, is a 

 very general one in organic forms, and is exhibited in various con- 

 ditions by both animals and plants. Its simplest expression is 

 found in cell-structure and the binary division of cells. It shows 

 itself as affecting larger masses of structure in the arborescent 

 colonies of Calentera, in the radial or antimeric composition of 

 Echinoderms and of Compound Ascidians, and in the linear or 

 metameric segmentation of Worms, Arthropods, and Vertebrates. 

 There is abundant evidence that this property is a general one, 

 which may assert itself at any period in the history of a group of 

 animals, and does not imply special unity of origin in forms which 

 exhibit it. As pointed out in the article HYDKOZOA, merogenesis 

 the name applicable to this phenomenon generally may take 

 an extreme and complete character, leading to the separation and 

 independence of the units of structure produced ; in that case it 

 may be termed eumerogenesis. Or the process may be very partial, 

 occurring only during a period of embryonic growth, and subse- 

 quently ceasing, so that later growth obscures or obliterates it 

 altogether (dysmerogenesis). There is no ground for assuming 

 that either one of these extremes is fundamental or original. Any 

 mechanical or nutritional condition may lead to merogenesis in an 

 organism in which the tissues have a certain reproductive capacity, 

 or have not acquired final differentiation ; and it will depend upon 

 the balance of advantage, determined by natural selection, whether 

 the segmentation (supposing the merogenesis to take the linear 

 form) results in the separation of segment-buds, or in the formation 

 of an annulate body, or leaves traces of its occurrence only in certain 

 tissues and organs. The Cestoid Worms present within the range 

 of a single group almost every grade of eumerogenesis and dysmero- 

 genesis (CaryophyllsBus, Ligula, Teenia). In the otherwise amero- 

 genetic' Mollusca, Chiton and the pearly Nautilus show dysmero- 

 genesis in certain organs, whilst the Planarian Worms frequently 

 exhibit eumerogenesis in their bud-segmentation (to be compared 

 with that of the Annelid Gtenodrilus described by Zeppelin, 9) and 

 the elongated Neruertines only slight traces of dysmerogenesis. 



If we deny Dohrn's assumption with reference to segmentation, 

 we are no longer led in the direction of the Annelids (Chsetopods) 

 in our search for the ancestry of the Craniate Vertebrata. 



The fact that the notochord is the forerunner of the segmented 

 vertebral column, and is itself never segmental, instead of being a 

 difficulty, acquires directive significance. The fact that the nerve 

 tube is dorsal, and not ventral, no longer requires the large assump- 

 tion that animals have reversed their habitual carriage, but suggests 

 that the Craniates' ancestor had a dorsal median nerve, which has 

 increased in size and importance so as to become the nerve-tube 

 of existing forms. 

 The explanation of 

 the curious struc- 

 ture of the brain 

 will have to be 

 found otherwise 

 than in the assump- 

 tion of a perforating 

 pharynx, an as- 

 sumption which the 

 recent discovery of 

 the true nature of 

 the pineal body has 

 rendered untenable 

 in the latest form 

 advocated by its 

 ingenious author, 

 whose speculations, 

 nevertheless, de- 

 serve the fullest re- 

 cognition as having 

 stimulated inquiry 

 and guided observa- 

 tion. 



Balfour (10) in 

 1878 refused to a- 

 dopt Dohrn's views, 

 and considered it 

 probable that the 

 dorsal position of 

 the nerve cord in 

 Vertebrata could be 

 accounted for, with- 

 out any assumption 

 of a substitution of 

 a pair of gill-slits for 

 the original mouth, 

 by assuming that 



f) AMPHIOXUS 



FIG. 6. Comparison of nervous systems of a Xemertine, 

 a prim itive Craniate, and A mphioxus. m, Median dor- 

 sal nerve, which becomes the myelon in the Craniate 

 and Amphioxus, acquiring an anterior enlargement in 

 the former ; I, lateral nerve (right and left), absent by 

 degeneration in Amphioxus ; lg, ganglia of lateral 

 nerve, forming a single large lobe on each side in 

 the Nemertine, and broken into a metameric series in 

 the Craniate ; v, roots of vagus nerve of the Craniate ; 

 dr, dorsal roots of nerves given off from myelon or 

 median dorsal nerve ; vr, ventral roots of these nerves, 

 here represented as separate nerves ; 0, mouth. (After 

 Hnbrecht.) 



assuming 



primitively the nerve-cord consisted of two lateral cords, as seen at 

 the present time in the Nemertine Worms, and that these cords 

 have coalesced dorsally in Vertebrata, just as it is clearly demon 



