Selachii

Essentially different from the wide-mouthed discoid gastrula of most of the selachii is the narrow-mouthed discoid gastrula (or epigastrula) of the amniotes, the reptiles, birds, and monotremes; between the two -- as an intermediate stage -- we have the amphigastrula of the amphibia. ❋ Ernst Heinrich Philipp August Haeckel (1876)

The eleventh stage is formed by the primæval fish or selachii (sharks); the twelfth by the mud fish, of which there still live the protopterus in Africa, the lepidosiren in the tributaries of the ❋ Rudolf Schmid (N/A)

But while in this case, as in the case of the amphibia, the small ovum completely divides into cells in segmentation, this is no longer so in the great majority of the selachii (or Elasmobranchii). ❋ Ernst Heinrich Philipp August Haeckel (1876)

The gastrulation of the primitive fishes or selachii (sharks and rays) has been carefully studied of late years by Ruckert, Rabl, and H.E. Ziegler in particular, and is very important in the sense that this group is the oldest among living fishes, and their gastrulation can be derived directly from that of the cyclostoma by the accumulation of a large quantity of food-yelk. ❋ Ernst Heinrich Philipp August Haeckel (1876)

Unequal total segmentation follows the same lines in the oldest fishes, the selachii and ganoids, which are directly descended from the cyclostoma. ❋ Ernst Heinrich Philipp August Haeckel (1876)

Among the fishes the selachii, or primitive fishes, yield the most important information on these and many other phylogenetic questions ❋ Ernst Heinrich Philipp August Haeckel (1876)

The recent investigations of Ruckert and Van Wijhe on the mesodermic segments of the trunk and the excretory system of the selachii show that these "primitive fishes" are closely related to the amphioxus in this further respect. ❋ Ernst Heinrich Philipp August Haeckel (1876)

The ideal figure of the vertebrate as given in Figures 1.98 to 1.102 is a hypothetical scheme or diagram, that has been chiefly constructed on the lines of the amphioxus, but with a certain attention to the comparative anatomy and ontogeny of the ascidia and appendicularia on the one hand, and of the cyclostoma and selachii on the other. ❋ Ernst Heinrich Philipp August Haeckel (1876)

Ruckert, this sexual structure is at first segmental even in the actual selachii, and the several gonotomes only blend into a simple sexual gland on either side secondarily. ❋ Ernst Heinrich Philipp August Haeckel (1876)

In some of the fishes (selachii) and in the cyclostoma we find six or seven of them permanently. ❋ Ernst Heinrich Philipp August Haeckel (1876)

In the wide-mouthed, circular discoid gastrula of the selachii or primitive fishes, which spreads quite flat on the large food-yelk, the anterior semi-circle of the border of the disk is the ventral, and the posterior semicircle the dorsal lip (Figure 1.119 ❋ Ernst Heinrich Philipp August Haeckel (1876)