VERTEBRATA 



177 



entirely disappeared (Cydostoma) (as has their anterior extension, 

 the opereulum, in many cases) or it has become, together with its 

 skeletal elements, concentrated in two regions forming the pectoral 

 and the pelvic paired appendages or limbs, with their respective 

 girdles. 



The cerebro-spinal nerve-centre and the disposition of the nerves 

 issuing from it present a remarkable complexity, and at the same 

 time uniformity, of structure in all Craniata (see fig. 5). The fore- 

 most of the three prim- _ 



ary cerebral vesicles _' >^. * A 



gives rise to paired an- 

 terior outgrowths, the 

 prosencephala, to a 

 median dorsal 

 growth, the 

 stalk of the 

 pineal eye 

 (rudimentary 

 in all exist- 

 ing Craniate), 

 and to a me- 

 dian ventral 

 outgrowth, 

 which is met 

 by an inva 

 gination 

 the epide 

 of the oral 

 cavity form- 

 ing the pitu- 

 itary body, 

 further, to a 

 pair of lateral 

 outgrowths, 

 which be- 

 come the 

 right and left 

 optic nerves 

 and retinae 

 respectively. 



of FIG- 5. Diagrams of Craniate brain. A. Embryonic condition 

 of neural tube. G, Cerebral portion ; E, spinal cord ; I, 

 II, III, three primary cerebral vesicles. B. Longitudinal 

 section of adult brain, applicable to any and every Craniate 

 Vertebrate. Be, Floor of skull ; C*, notochord ; SD, roof or 

 skull ; If Hi, nasal cavity ; VH, fore-brain, prosencephalon, 

 or cerebrum ; (Af, olfactory lobe ; C, corpus striatum ; ZH, 

 thalamencephalon, corresponding to primary anterior vesicle, 

 from which the prosencephalon has grown out as well as Z, 

 the epiphysis or pineal body ; I, infnndibulum, with attached 

 hypophysis or pituitary body H; Opt, optic nerve; Tko, 

 thalamus options ; HC, posterior commissure ; MH, mesen- 

 cephalon (corpora quadrigmina or optic lobes or mid-brain); 

 HH, cerebellum or metencephalon ; SH, medulla oblongata 

 or epencephalon (the reference line touches the membranous 

 roof of the so-called "fourth ventricle" of the brain); Co, 

 canal of the spinal cord or myelon. (From Wiedersheim.) 



The modifications of the hindmost of the three prim- 

 ary vesicles are also extremely definite and persistent throughout 

 the group: its anterior dorsal surface enlarges and becomes the 

 cerebellum (the metencephalon), whilst the cavity of its hinder 

 part (the medulla oblongata or the epencephalon) becomes compara- 

 tively wide, and is covered dorsally by a thin membrane only, in 

 which nervous tissue does not take a part. The intermediate 

 primary cerebral vesicle (the mesencephalon) does not give rise to 

 outgrowths. 



In all Craniata nerves are given off from the cerebro-spinal cord 

 or tube with great regularity, one right and left in each success- 

 ive myomere or segment of the body -wall. Each nerve has two 

 roots, a dorsal (sensory) and a ventral (motor). A commissure 

 between each successive pair of deep or intestinal branches of the 

 spinal nerves forms the so-called sympathetic nerve-cords, one on 

 each side of the vertebral column. Nerves similar to the spinal 

 nerves, but not identical with them, are given off from the brain, 

 and perforate the cranial box right and left. In all Craniata there 

 are ten pairs of nerves which originate thus, and in the higher 

 forms two more pairs (elsewhere spinal) are included amongst those 

 which thus perforate the cranium (spinal accessory and hypoglossal). 

 The order and character of the cranial nerves are the same in all 

 Craniata. The first (olfactory) and the second (optic) are unlike 

 spinal nerves in both distribution and origin. As we pass back- 

 wards along the series, the cranial nerves are found to resemble more 

 and more the ordinary spinal nerves. Hence it has been inferred 

 that the cranial region consisted at one time of a number of distinct 

 myomeres (as many as uine), which have become fused and modi- 

 fied to form the typical craniate "head." The oculo-motor or 3d 

 nerve indicates the first of these segments, the trochlear or 4th 

 nerve the second, the abducens or 6th the third, the facial and 

 auditory (7th and 8th) the fourth, the glosso-pharyngeal or 9th the 

 fifth, and the vagus or 10th, with certain of its branches supposed 

 to have been originally distinct nerves, the sixth, seventh, eighth, 

 and ninth. It is probable that in Craniata the metamerism of the 

 gill-slits does not correspond to the metamerism of the body-wall. 

 The mouth and each successive gill-slit are related to a bifurcate 

 branch of a cranial nerve (furcal nerve) in lower Craniata (see fig. 

 3) ; these furcal nerves do not correspond, so far as we can at present 

 judge, with cranial myomeres. 



The lateral pair of eyes (as opposed to the rudimentary pineal or 

 parietal eye) present a striking uniformity of origin and structure 

 throughout Craniata. Not only are they uniformly developed from 

 three elements, viz., the retinal cup which grows out from the 

 anterior of the primary cerebral vesicles, the epidermal lens which 

 grows inwards from the surface of the skin, and the connective 

 tissue between these two, but we find that the muscles attached to 

 the eyeball are identical throughout the series : that is, the superior, 



inferior, and internal rectos, and the inferior obliqne mnscles repre- 

 sent the first cranial myomere, the superior oblique represent 

 the second, and the external rectus represent the third cranial 

 myomere. 



The olfactory sacs are paired in all except Cydostoma, in which 

 they are represented by a single sac which may or may not be 

 archaic in its azygos character. The auditory sacs are paired 

 organs which develop as invaginations from the surface, the orifice 

 of invagination closing up, at the hinder part of the cranial region. 

 They present a gradually increasing complexity of form as we pass 

 from aquatic to terrestrial forms, but are identical in essential struc- 

 ture throughout. 



All Craniata, except some Fishes, possess a muscular process on 

 the floor of the oral cavity which may carry teeth, or act as a lick- 

 ing organ, or assist in suction. This is the tongue. 



All Craniata, with degradational exceptions, possess an outgrowth, 

 single or paired, of the post-pharyngeal region of the alimentary 

 canal, which is filled with gas. In many Fishes this becomes shut 

 off from the gut ; in others it remains in communication with the 

 gut by an open duct In Fishes it functions as a hydrostatic appar- 

 atus. In terrestrial Craniata it is subservient to the gas-exchange 

 of the blood and becomes the lungs. 



All Craniata have a large and compact liver ; and a pancreas is 

 also uniformly present, except in Cydostoma, some bony Fishes, and 

 the lower Amphibia. 



All Craniata have a thick-walled muscular heart, which appears 

 first as an "atrium," receiving the great veins, attached to a 

 "ventricle" by which the blood received from the atrium is pro- 

 pelled through a number of arteries, right and left, corresponding in 

 number to the pharyngeal gill-slits between which they pass. 

 A prseatrial chamber (the sinus) and an extra-ventricular chamber 

 (the conns) are added to the primitive chambers ; but the most 

 important modifications arise in consequence of the development 

 of pulmonary respiration and the gradual separation of the cavities 

 of the heart by median septa into a double series, a right and a left 

 The plan of the great arteries in all Craniata is in origin the same, 

 and is determined by the primitive existence of a branchial circula- 

 tion in the gill-slits, which is obliterated in higher forms. Simi- 

 larly the plan of the great veins is identical, the primitive posterior 

 vertebral veins of lower Craniata, though persistent in higher 

 members of the group, having their function gradually usurped by 

 the excessive development of the renal vein, and of renal-portal and 

 ultimately of iliac veina. 



All Craniata have a lymphatic system or series of channels by 

 which the exudation from the capillary blood-vessels is returned 

 to the vascular system. It includes in its space-system the coelom 

 and a variety of irregular and canalicular spaces in the connective 

 tissues. Masses of spongy tissue (adenoid tissue, lymphatic glands) 

 exist, through which the lymph filters, and there acquires corpus- 

 cular elements as well as chemical elaboration. At various points 

 in various Craniata pulsating or simple communications are estab- 

 lished, between the lymphatic system and the veins. A special 

 and characteristic communication is established in the spleen, an 

 organ which is found in all Craniata, either as a single mass or as 

 scattered masses of spongy tissue in which blood-vessels and lymph- 

 atics unite. 



The renal organs of Craniata are primitively a series of nephridia 

 corresponding in number to the myotomes of the mid-region of the 

 body in which they exist. They are connected in the simplest 

 Craniata by a right and a left archinephric duct, which appear to 

 be in origin lateral grooves of the epidermal surface. This primi- 

 tive renal system has been modified in some lower forms (Cydostoma 

 and Teleostean Fishes) by the atrophy of its anterior portion. But 

 in all other Craniata it acquires relations to the gonads or ovary 

 and testes, so that an anterior portion of the archinephros and a 

 corresponding longitudinal tract of the duct become separated to 

 serve as oviduct, a middle portion to serve as sperm-duct, while a 

 posterior portion retains exclusively or shares with the middle 

 portion the function of urinary excretion. The male and female 

 gonads are, with the rarest exceptions, developed in distinct indi- 

 viduals, though the rudiments of the suppressed gonad may in 

 some cases (Amphibia) be traceable in either sex. 



The group of Craniate Vertebrata thus anatomically described, Sub- 

 whilst retaining the essential unity indicated, presents an immense divisions 

 variety of modifications. The chief modifications are distinctly of Crowi- 

 traceable to and accounted for by mechanical and physiological ata. 

 adaptation to a terrestrial and air-breathing life, as opposed to the 

 earlier aquatic and branchial condition. The existing forms of 

 Craniata have been arrested at several points, in the progress towards 

 the most extreme adaptation to terrestrial conditions, which is 

 presented by those forms that can not only breath air and live on 

 dry ground but fly habitually in the air. The organs most obviously 

 affected by this progressive adaptation are the skin, the skeleton, 

 especially of the limbs, the pharyngeal gills, and the air-bladder. 

 This fact will appear most clearly in the subjoined classification of 

 Craniata ; for space does not permit us to pursue further the 

 history of these modifications. 



