The Evolution of Man, V.2, Ernst Haeckel [free ebook reader for pc txt] 📗

of the former in existence now, but they are very interesting, because in their whole structure they stand midway between the Acrania and the Gnathostoma. They are much more advanced than the Acrania, much less so than the fishes, and thus form a very welcome connecting-link between the two groups. We may therefore consider them a special intermediate group, the fourteenth and fifteenth stages in the series of our ancestors.

(FIGURE 2.247. The large marine lamprey (Petromyzon marinus), much reduced. Behind the eye there is a row of seven gill-clefts visible on the left, in front the round suctorial mouth.)

The few surviving species of the Cyclostoma are divided into two orders--the Myxinoides and the Petromyzontes. The former, the hag-fishes, have a long, cylindrical, worm-like body. They were classed by Linne with the worms, and by later zoologists, with the fishes, or the amphibia, or the molluscs. They live in the sea, usually as parasites of fishes, into the skin of which they bore with their round suctorial mouths and their tongues, armed with horny teeth. They are sometimes found alive in the body cavity of fishes (such as the torsk or sturgeon); in these cases they have passed through the skin into the interior. The second order consists of the Petromyzontes or lampreys; the small river lamprey (Petromyzon fluviatilis) and the large marine lamprey (Petromyzon marinus, Figure 2.247). They also have a round suctorial mouth, with horny teeth inside it; by means of this they attach themselves by sucking to fishes, stones, and other objects (hence the name Petromyzon = stone-sucker). It seems that this habit was very widespread among the earlier Vertebrates; the larvae of many of the Ganoids and frogs have suctorial disks near the mouth.

The class that is formed of the Myxinoides and Petromyzontes is called the Cyclostoma (round-mouthed), because their mouth has a circular or semi-circular aperture. The jaws (upper and lower) that we find in all the higher Vertebrates are completely wanting in the Cyclostoma, as in the Amphioxus. Hence the other Vertebrates are collectively opposed to them as Gnathostoma (jaw-mouthed). The Cyclostoma might also be called Monorhina (single-nosed), because they have only a single nasal passage, while all the Gnathostoma have two nostrils (Amphirhina = double-nosed). But apart from these peculiarities the Cyclostoma differ more widely from the fishes in other special features of their structure than the fishes do from man. Hence they are obviously the last survivors of a very ancient class of Vertebrates, that was far from attaining the advanced organisation of the true fish. To mention only the chief points, the Cyclostoma show no trace of pairs of limbs. Their mucous skin is quite naked and smooth and devoid of scales. There is no bony skeleton. A very rudimentary skull is developed at the foremost end of their chorda. At this point a soft membranous (partly turning into cartilage), small skull-capsule is formed, and encloses the brain.

The brain of the Cyclostoma is merely a very small and comparatively insignificant swelling of the spinal marrow, a simple vesicle at first. It afterwards divides into five successive cerebral vesicles, like the brain of the Gnathostoma. These five primitive cerebral vesicles, that are found in the embryos of all the higher vertebrates from the fishes to man, and grow into very complex structures, remain at a very rudimentary stage in the Cyclostoma. The histological structure of the nerves is also less advanced than in the rest of the vertebrates. In these the auscultory organ always contains three circular canals, but in the lampreys there are only two, and in the hag-fishes only one. In most other respects the organisation of the Cyclostoma is much simpler--for instance, in the structure of the heart, circulation, and kidneys. We must especially note the absence of a very important organ that we find in the fishes, the floating-bladder, from which the lungs of the higher Vertebrates have been developed.

When we consider all these peculiarities in the structure of the Cyclostoma, we may formulate the following thesis: Two divergent lines proceeded from the earliest Craniota, or the primitive Craniota (Archicrania). One of these lines is preserved in a greatly modified condition: these are the Cyclostoma, a very backward and partly degenerate side-line. The other, the chief line of the Vertebrate stem, advanced straight to the fishes, and by fresh adaptations acquired a number of important improvements.

(FIGURE 2.248. Fossil Permian primitive fish (Pleuracanthus Dechenii), from the red sandstone of Saarbrucken. (From Doderlein.) I Skull and branchial skeleton: o eye-region, pq palatoquadratum, nd lower jaw, hm hyomandibular, hy tongue-bone, k gill-radii, kb gill-arches, z jaw-teeth, sz gullet-teeth, st neck-spine. II Vertebral column: ob upper arches, ub lower arches, hc intercentra, r ribs. III Single fins: d dorsal fin, c tail-fin (tail-end wanting), an anus-fin, ft supporter of fin-rays. IV Breast-fin: sg shoulder-zone, ax fin-axis, ss double lines of fin-rays, bs additional rays, sch plates. V Ventral fin: p pelvis, ax fin-axis, ss single row of fin-rays, bs additional rays, sch scales, cop penis.

FIGURE 2.249. Embryo of a shark (Scymnus lichia), seen from the ventral side, v breast-fins (in front five pairs of gill-clefts), h belly-fins, a anus, s tail-fin, k external gill-tuft, d yelk-sac (removed for most part), g eye, n nose, m mouth-cleft.)

The Cyclostoma are almost always classified by zoologists among the fishes; but the incorrectness of this may be judged from the fact that in all the chief and distinctive features of organisation they are further removed from the fishes than the fishes are from the Mammals, and even man. With the fishes we enter upon the vast division of the jaw-mouthed or double-nosed Vertebrates (Gnathostoma or Amphirhina). We have to consider the fishes carefully as the class which, on the evidence of palaeontology, comparative anatomy, and ontogeny, may be regarded with absolute certainty as the stem-class of all the higher Vertebrates or Gnathostomes. Naturally, none of the actual fishes can be considered the direct ancestor of the higher Vertebrates. But it is certain that all the Vertebrates or Gnathostomes, from the fishes to man, descend from a common, extinct, fish-like ancestor. If we had this ancient stem-form before us, we would undoubtedly class it as a true fish. Fortunately the comparative anatomy and classification of the fishes are now so far advanced that we can get a very clear idea of these interesting and instructive features.

In order to understand properly the genealogical tree of our race within the vertebrate stem, it is important to bear in mind the characteristics that separate the whole of the Gnathostomes from the Cyclostomes and Craniota. In these respects the fishes agree entirely with all the other Gnathostomes up to man, and it is on this that we base our claim of relationship to the fishes. The following characteristics of the Gnathostomes are anatomic features of this kind: (1) The internal gill-arch apparatus with the jaw arches; (2) the pair of nostrils; (3) the floating bladder or lungs; and (4) the two pairs of limbs.

The peculiar formation of the frame work of the branchial (gill) arches and the connected maxillary (jaw) apparatus is of importance in the whole group of the Gnathostomes. It is inherited in rudimentary form by all of them, from the earliest fishes to man. It is true that the primitive transformation (which we find even in the Ascidia) of the fore gut into the branchial gut can be traced in all the Vertebrates to the same simple type; in this respect the gill-clefts, which pierce the walls of the branchial gut in all the Vertebrates and in the Ascidia, are very characteristic. But the EXTERNAL, superficial branchial skeleton that supports the gill-crate in the Cyclostoma is replaced in the Gnathostomes by an INTERNAL branchial skeleton. It consists of a number of successive cartilaginous arches, which lie in the wall of the gullet between the gill-clefts, and run round the gullet from both sides. The foremost pair of gill-arches become the maxillary arches, from which we get our upper and lower jaws.

The olfactory organs are at first found in the same form in all the Gnathostomes, as a pair of depressions in the fore part of the skin of the head, above the mouth; hence, they are also called the Amphirhina ("double-nosed"). The Cyclostoma are "one-nosed" (Monorhina); their nose is a single passage in the middle of the frontal surface. But as the olfactory nerve is double in both cases, it is possible that the peculiar form of the nose in the actual Cyclostomes is a secondary acquisition (by adaptation to suctorial habits).

A third essential character of the Gnathostomes, that distinguishes them very conspicuously from the lower vertebrates we have dealt with, is the formation of a blind sac by invagination from the fore part of the gut, which becomes in the fishes the air-filled floating-bladder. This organ acts as a hydrostatic apparatus, increasing or reducing the specific gravity of the fish by compressing or altering the quantity of air in it. The fish can rise or sink in the water by means of it. This is the organ from which the lungs of the higher vertebrates are developed.

(FIGURE 2.250. Fully developed man-eating shark (Carcharias melanopterus), left view. r1 first, r2 second dorsal fin, s tail-fin, a anus-fin, v breast-fins, h belly-fins.)

Finally, the fourth character of the Gnathostomes in their simple embryonic form is the two pairs of extremities or limbs--a pair of fore legs (breast-fins in the fish, Figure 2.250 v) and a pair of hind legs (ventral fins in the fish, Figure 2.250 h). The comparative anatomy of these fins is very interesting, because they contain the rudiments of all the skeletal parts that form the framework of the fore and hind legs in all the higher vertebrates right up to man. There is no trace of these pairs of limbs in the Acrania and Cyclostomes.

Turning, now, to a closer inspection of the fish class, we may first divide it into three groups or sub-classes, the genealogy of which is well known to us. The first and oldest group is the sub-class of the Selachii or primitive fishes; the best-known representatives of which to-day are the orders of the sharks and rays (Figures 2.248 to 2.252). Next to this is the more advanced sub-class of the plated fishes or Ganoids (Figures 2.253 to 2.255). It has been long extinct for the most part, and has very few living representatives, such as the sturgeon and the bony pike; but we can form some idea of the earlier extent of this interesting group from the large numbers of fossils. From these plated fishes the sub-class of the bony fishes or Teleostei was developed, to which the great majority of living fishes belong (especially nearly all our river fishes). Comparative anatomy and ontogeny show clearly that the Ganoids descended from the Selachii, and the Teleostei from the Ganoids. On the other hand, a collateral line, or rather the advancing chief line of the vertebrate stem, was developed from the earlier Ganoids, and this leads us through the group of the Dipneusta to the important division of the Amphibia.

(FIGURE 2.251. Fossil angel-shark (Squatina alifera), from the upper Jurassic at Eichstatt. (From Zittel.) The cartilaginous skull is clearly seen in the broad head, and the gill-arches behind. The wide breast-fin and the narrower belly-fin have a number of radii; between these and the vertebral column are a number of ribs.)

The earliest fossil remains of Vertebrates that we know were found in the Upper Silurian (

Chapter 2.

18), and belong to two groups--the Selachii and the Ganoids. The most primitive of all known representatives of the earliest fishes are probably the remarkable Pleuracanthida, the genera Pleuracanthus, Xenacanthus, Orthocanthus, etc. (Figure 2.248). These ancient cartilaginous fishes agree in most points of structure with the real sharks (Figures 2.249 and 2.250); but in other respects they seem to be so much simpler in organisation that many palaeontologists separate them altogether, and regard them as Proselachii; they are probably closely related to the extinct ancestors of the Gnathostomes.