154 



PISCES. 



B 



their skin is without scales. They have no paired fins but the system 

 of vertical fins is developed over the whole length of the dorsal 

 surface and of the tail, and is usually supported by cartilaginous 

 rays. The skeleton is confined to a cartilaginous rudiment of the 

 vertebral column and skull. The notochord persists as the axial 

 skeleton : its sheath presents traces of segmentation in the presence 

 of rudimentary cartilaginous neural arches (fig. 600, I), and in the 

 caudal region (Petromyzon) of the lower vertebral arches also. 



At the anterior end of the notochord there is a cartilagino- 

 membranous cranial capsule enclosing the brain. It has a bony 'basal 



region and lateral cartilaginous 

 vesicles in which the auditory 

 organs are enclosed (fig. 600). 

 In place of the visceral skeleton 

 there are cartilaginous pieces 

 surrounding the palate and pha- 

 rynx, various labial cartilages 

 and a complicated frame work 

 of cartilaginous rods, which 

 form the so-called branchial 

 basket round the branchial sacs, 

 and are in part attached to the 

 vertebral column. 



The Cydostomi possess a brain 

 of the piscine type with three 

 principal sense nerves and a 

 reduced number of spinal-like 

 nerves. Two eyes are always 

 present, but they may be hidden 

 under the skin or even covered 

 by muscles (Myxine, larva of Petromyzon}. The olfactory organ 

 is an unpaired sac opening in the median line between the eyes. In 

 the Myxinoids the olfactory capsule has in addition a posterior 

 opening which pierces the palate and can be closed by a valvular 

 apparatus. This communication between the nasal and pharyngeal 

 cavities serves for the introduction of water into the branchial sac : 

 for the mouth when performing its function as a suctorial organ is 

 closed so far as the passage of water is concerned. The auditory 

 organ is reduced to a simple membranous labyrinth which consists 

 of the vestibulum and one or two semicircular canals. 



Alimentary canal. The mouth, which is surrounded by fleshy 



FIG. 600. Skull and beginning of the verte- 

 bral column of Petromyzon mnrin/m (after 

 Joh. Miiller). a, In longitudinal vertical 

 section. I, Seen from above. A, noto- 

 chord ; -B, neural canal ; C, rudimentary 

 vertebral arches ; D, cartilaginous part, 

 and D', membranous part of the cranial 

 roof ; .E, base of skull ; F, auditory cap- 

 sule; G, nasal capsule; <?', naso-palatiiie 

 duct ; Or, blind end of G' ; H, process of 

 the bony palate ; J, posterior plate cover- 

 ing- the mouth ; K, anterior plate covering 

 the mouth ; L, labial ring ; M. styliform 

 appendage of L. 



