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ANATOMY]
255
ICHTHYOLOGY


pyloric valve. In the lower gnathostomatous fishes (Selachians, Crossopterygians, Dipnoans, sturgeons) the intestine possesses the highly characteristic spiral valve, a shelf-like projection into its lumen which pursues a spiral course, and along the turns of which the food passes during the course of digestion. From its universal occurrence in the groups mentioned we conclude that it is a structure of a very archaic type, once characteristic of ancestral Gnathostomata; a hint as to its morphological significance is given by its method of development! In an early stage of development the intestinal rudiment is coiled into a spiral and it is by the fusion together of the turns that the spiral valve arises. The only feasible explanation of this peculiar method of development seems to lie in the assumption that the ancestral gnathostome possessed an elongated coiled intestine which subsequently became shortened with a fusion of its coils. In the higher fishes the spiral valve has disappeared#-l:”'ng still found, however, in, a reduced condition in Amia and Lfpidosleus, and possibly as a faint vestige in one or two Teleosts (certain Clupeidaw and Salmonidaea). In the majority of the Teleosts the absence of spiral valves is coupled with a secondary elongation of the intestinal region, which in extreme cases (Loricariidae) may be accompanied by a secondary spiral coiling. The terminal part of the alimentary canal-the cloaca-is characterized by the fact that into it open the two kidney ducts. In Teleostomes the cloaca is commonly flattened out, so that the kidney ducts and the alimentary canal come to open independently on the outer surface.

The lining of the alimentary canal is throughout the greater part of its extent richly glandular. And at certain points local enlargements of the secretory surface take place so as to form glandular diverticula. The most ancient of these as indicated by its occurrence even in Arnphioxus appears to be the liver, which, originally-as we may assume-mainly a digestive gland, has in the existing Craniates developed important excretory and glycogen-storing functions. Arising in the embryo as a simple caecum, the liver becomes in the adult a compact gland of very large size, usually bi-lobed in shape and lying in the front portion of the splanchnocoele. The stalk of the liver rudiment becomes drawn out into a tubular bile duct, which may become subdivided into branches, and as a rule develops on its course a pocket-like expansion, the gall-bladder. This may hang freely in the splanchnocoele or may be, as in many Selachians, imbedded in the liver substance. °

The pancreas also arises by localized bulging outwards of the intestinal lining-there being commonly three distinct rudiments in the embryo. In the Selachians the whitish compact pancreas of the adult opens into the intestine some little distance behind the opening of the bile duct, but in the Teleostomes it becomes involved in the liver outgrowth and mixed with its tissue, being frequently recognizable only by the study of microscopic sections. In the Dipnoans the pancreatic rudiment remains imbedded in the wall of the intestine: its duct is united with that of the liver.

Pylaric Caeca.-In the Teleostomi one or more glandular diverticula commonly occur at the commencement of the intestine and are known as the pyloric caeca. There may be a single caecum (crossopterygians, Ammodyles amongst Teleosts) or there may be nearly two hundred (mackerel). In the sturgeons the numerous caeca form a compact gland. In several families of Teleosts, on the other hand, there is no trace of these pyloric caeca. .

In Selachians a small glandular diverticulum known as the rectal gland opens into the terminal part of the intestine on its dorsal side.

Coelomic Organs.-The development of the mesoderm in the restricted sense (mesotheliurn) as seen in the fishes (lamprey, Lepidosiren, Proloplerus, Polyplerus) appears to indicate beyond I. Rixckert, Arch. Entwickelungsmech. Band iv., 1897, S. 298; J. Graham Kerr, Phil. Trans. B. 192, 1900, p. 325, and The Budgett Memorial Volume.

2 Cuvier et Valenciennes, 'Hist. nat. des poiss. xix., 1846, p. 151. 5 ]. Rathke, Ub. d. Darmkanal u.s.'w. d. Fische, Halle, 1824, S. 62. 1

4

doubt that the mesoderm segments of vertebrates are really enterocoelic pouches in which the development of the lumen is delayed. Either the inner, or both inner and outer (ag. Lepidosiren) walls of the mesoderm segment pass through a myoepithelial condition and give rise eventually to the great muscle segments (myomeres, or myotomes) which lie in series on each side of the trunk. In the fishes these remain distinct throughout life. The fins, both median and paired, obtain their musculature by the ingrowth into them of muscle buds from the adjoining myotomes.

Electrical Organs!-It is characteristic of muscle that at the moment of contraction it produces a slight electrical disturbance. In certain fishes definite tracts of the musculature show a reduction of their previously

predominant

function of contrac- 9, oe

tion and an increase V f '%

of their previously #1 » 541, f=~o> — I - ' ' ', ' 'subsidiary

function of in / -"f

producing electrical V, ” ' "JI disturbance; so that “" il, "@; WI “H 1 . ~~> ~¢<~, -' r~€1;Y~= 'R

the latter function is, e ar/

now predominant. " ;g ~ § $ff/l,

In the skates (Raia) “fig l, / N” the electrical organ is

a fusiform structure

derived from the lateral '-.

mu sculpture of the ' bf

tail; in Gymrmtus-the

electric eel-and in

Mormyrus it forms an

enormous structure

occupying the place of

the ventral halves of 'V

the myotomes along .

nearly the whole length

of the body; in Tor- Q —/,

pedo it forms a large, V

somewhat kidney- *

3/'lg§ 'vl3;g fsggturilbogg From Gegenhaur, Unlemwhungen zur 1/erglerlsh. lying on each side of the

head and derived from

the musculature of the

Anal. der Wirbeltiere, by permission of Wilhelm Engelmann.

FIG. 10.-View of Torpedo from the dorsal anterior visceral arches side: the electric organs are exposed. In Torpedo the nerve- 1»F°f@l>fa“supply is derived from H1 Mesencephalom

cranial nerves VII. IX. IH' Cerebquum and the anterior bran- IV' 1€@<=tf1C l°be', phia, branches of X br, Lommorr muscular sheath covering The electric organ

is composed of prismatic

columns each

built up of a row of

compartments. Each

compartment contains

a lamellated electric

f.

a.e,

0,

l.

bronchial clefts (on the left side this has been removed so as to expose the series of bronchial sacs).

Spiracle.

Electric organ, on the left side the nerve-supply is shown.

Eye.

Sensory tubes of lateral line system. disc representing the shortened-up and otherwise metamorphosed muscle fibre. On one face (ventral in Torpedo, anterior in Raia) of the electric disc is a gigantic end-plate supplied by a beautiful, dichotomously branched, terminal nervous arborization.

The development of the mesoderm of the head region is too obscure for treatment here.5 The ventral portion of the trunk mesoderm gives rise to the splanchnocoel or general coelom. Except in the Myxinoids the anterior part of the splanchnocoel becomes separated off as a pericardia cavity, though in adult Selachians the separation becomes incomplete, the two cavities being in communication by a pericardia-peritoneal canal. Nephridial Systemr-The kidney system in fishes consists of segment ally arranged tubes leading from the coelom into a longitudinal duct which opens within the hinder end of the enteron-the Whole forming what is known as the archinephros (Lankester) or holonephros (Price). Like the other segmented 4 Cf. W. Biedermann, Electro-Physiology. “Literature in N. K. Koltzoff, Bull. Soc. Nat. Moscou, 1901,

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