SKATE. 273. 



PLATE IV. 



SKATE (Raja Balis). 



THIS plate, illustrating the anatomy of the Skate, is introduced for the 

 urpose 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 (mandi- 

 bular arch) and the Hyoidean arch the partial homologue of the Eusta- 

 chian tube which opens from the mouth behind the eye : the minute 

 apertures of the aquaeductus vestibuli, or pedicles of imagination 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, M tiller'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 form- 

 ing vasa deferentia, and the claspers placed to the inner side of the two 

 lobes of the ventral fins. 



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. 



T 



