THIS plate, illustrating the anatomy of the Skate, is introduced for the purpose of supplementing the description of the Perch, a member of the order Teleostei, p. 83. The Skate belongs to the more generalised order Elasmobranchii. In addition to points characteristic of this order detailed below, note in the examination of a specimen the following: - the exo-skeletal spines on the dorsal surface resembling teeth, not only in form but in structure, and attached to a basal plate of bone: the great extent of naked skin: the spiracle or visceral cleft between Meckel's arch (mandibular arch) and the Hyoidean arch - the partial homologue of the Eustachian tube - which opens from the mouth behind the eye: the minute apertures of the aquaeductus vestibuli, or pedicles of invagination of the inner ear, placed at the posterior and dorsal aspect of the cranium, one on either side: the skeleton, cartilaginous with the exception of the bodies of the vertebrae: the communication between the pericardial and abdominal cavities in the shape of a bifurcated canal: the presence of a posterior division of the kidney or metanaphros, with a certain number of ureters passing off from the inner side of the organ to open into the dilatation, at the posterior end of the Wolffian or mesonephric duct: the union of a certain number of these ureters into a single duct, especially in the male: in the female the open conjoined mouths, situated ventrally at the root of the liver, of the two oviducts, Muller's ducts or Fallopian tubes; the 'nida-mental' gland situated on each oviduct; the thicker posterior uterine portions of the ducts: the two oviducal openings, one on each side, into the cloaca: in the male the connection of each testis to the anterior part of the corresponding Wolffian body or mesonephros, thus forming an epididymis; the convoluted anterior portions of the Wolffian or mesonephric ducts forming vasa deferentia, and the claspers placed to the inner side of the two lobes of the ventral fins.
Plate IV. Skate, Raja Batis.
The Rays are peculiar in the slight development of the azygos system of fins, which is restricted to small lobes on the dorsal side of the extremity of the tail: in the enormous expansion forwards, outwards, and backwards, of the pectoral fins which gives the body its great width; and in the bilobed form of the ventral fins. The skin of one Ray at least possesses the minute close-set denticles which constitute shagreen. The degree of development and the arrangement of the large cutaneous spines varies much in different species. The praenasal cartilage is always large, often extremely prolonged, and forms the pointed anterior extremity of the body.
FIG. 1. Skate, Raja Batis, female, ventral view from a specimen, dissected so as to show the heart, gills, and digestive tract in situ.
a. The line points to the spot where the conus arteriosus springs from the ventricle. This structure lies in front, with the auricle and ductus Cuvieri behind (see Fig. 2, infra).
b. The line points to the base of the ventral aorta at the spot where it springs from the conus, and gives off the two posterior innominate arteries. Each of these vessels divides into three branches - the three posterior branchial arteries which run on the outer side of the three posterior (II-IV) branchial arches and supply with venous blood the two gill-laminae, anterior and posterior, borne by each arch. These laminae, together with the fibrous septum which supports them and is continued to the skin separating the so-called gill-pouches inter se, are seen on the right side of the diagram. c. The line points to the anterior termination of the ventral aorta where it gives off the right and left anterior innominate arteries. Each of these arteries divides into the two anterior branchial arches which supply - the anterior, the single gill-lamina (=opercular gill of Ganoidei, pseudobranch of Teleostei) borne upon the posterior aspect of the hyoidean arch; the posterior, the two gill-laminae borne by the first branchial arch (I). The fifth branchial arch in the Rays and most Sharks, as in Teleostei and Ganoidei, bears no gill-laminae. The mode in which the branchial arteries arise from the aorta is characteristic of Rays.
d. The first of the five external gill-slits. The remaining four are seen arranged in a curved line behind. Gill-slits uncovered by an opercular fold are characteristic of all Sharks and Rays: their completely ventral position, of the latter only.
e. The aperture into the olfactory pit. This aperture is placed ventrally in nearly all Elasmobranchii. A groove leads from the pit to the corresponding angle of the mouth. Such a groove exists in the em-bryoes of all higher Vertebrata. The outer edge of the groove represents the fronto-nasal, the inner edge the praemaxillary, processes seen in the embryoes of Vertebrata which possess praemaxillary and maxillary bones. If the roof of the olfactory pouch is examined in a specimen it will be seen to possess two series of transverse folds.
f. The line rests upon the upper jaw, which is cartilaginous, and represents, as in all Elasmobranchii, a palato-pterygo-quad rate cartilage. The transverse slit of the mouth and the under jaw (=distal end of Meckel's cartilage) are seen with the rows of diamond-shaped teeth set edge to edge. The retention of a ventrally placed mouth is characteristic of the Elasmobranchii.
g. Jelly tubes or sensory ampullae. Only a certain number of these structures which are peculiar to Elasmobranchii have been figured. They run towards the head, where their inner ends are situated in close contact. Their length varies, the most anterior being short, the posterior long. Their apertures are easily found. The tubes are dilated at their inner extremities and possess in this species numerous lateral saccules. The nerves pass along the septa between these saccules, radiating from the centre. They form a plexus .in the walls of the saccules, the ultimate fibrils of which are probably continuous with hair cells in the lining epithelium. The jelly filling these tubes is firm, and is secreted by goblet cells scattered on the walls. The lateral line is not indicated in this figure. See Merkel, Endigun-gen der Sensibeln Nerven, &c; Rostock, 1880, p. 33, P1. II. Fig. 10; and Leydig, Beitrage zur Mikr. Anat. etc, der Rochen und Haien, Leipzig, 1852, p. 37. The ampullae are also described by the same authors, see Merkel, p. 43; Leydig, p. 41.