PAIRED FINS NERVOUS SYSTEM. 137 



part (ceratobranchial) remains and forms the so-called inferior 

 pharyngeal bones (pharyngealia inferiora). The upper segments of 

 the branchial arches, which are applied to the base of the skull, are 

 distinguished as the superior pharyngeal bones (pharyngobranchials 

 or pharyngealia superiora). 



Paired Fins.* The pectoral fins are in the Teleosteans attached to 

 the skull by means of the shoulder girdle. In the cartilaginous 

 fishes the shoulder girdle is a simple cartilaginous arch, which unites 

 with that of the other side in the middle ventral line. In the 

 cartilaginous Ganoids the shoulder-girdle is transitional between this 

 primary form and the secondary form, which is characteristic of the 

 Teleosteans (fig. 586), inasmuch as membrane bones (clavicle) are 

 applied to the primary cartilaginous girdle. Ossifications also arise 

 in the cartilage itself and give rise to bones known as the scapula 

 and coracoid, or the prsecoracoid. 



The skeleton of the fins, which is articulated to the shoulder- 

 girdle, can be derived from the primitive form of fin known as 

 the archipterygium, which still persists in Geratodus as an axial row 

 of cartilaginous pieces beset with jointed lateral rays (radii). 



The nervous system (fig. 588) presents the lowest and simplest 

 form found in any Vertebrate. In general the brain is small and 

 consists of several swellings lying one behind another. Of these the 

 small anterior, as the lobi olfactorii, pass into the olfactory nerves. 

 The larger anterior lobes correspond to the hemispheres, the median 

 globular swellings to the lobe of the third ventricle with the corpora 

 quadrigemina. From this part of the brain the optic nerves are 

 given off anteriorly, while on its lower surface the inf undibulum, to 

 which the pituitary body is attached, arises from the floor of the 

 third ventricle. 



The posterior region corresponds to the cerebellum and the medulla 

 oblongata. The cerebellum, which varies considerably in size and 

 form, constitutes a transverse bridge, which covers the anterior part 

 of the fourth ventricle. Lateral swellings the so-called lobi pos- 

 teriores are often developed in this region ; in the Sturgeons and 

 Squalidse at the origin of the trigeminal nerve, as the lobi nervi 

 trigemini; in Torpedo as the large lobi electrici, projecting over the 

 fourth ventricle. 



A separate visceral (sympathetic) nervous system is absent in the 



* Compare C. Gegenbaur, " Untersuchungen zur vergleichenden Anatomie 

 der Wirbelthiere." 2 Heft, Leipzig, 1865. 



C. Gegenbaur, "Ueber das Skelet der Gliedmassen." Jen. naturrviss. Zeitscli,, 

 Tom. V. 



