176 



THE SPINY DOGFISH— THE SKATE 



Third Day 



E. The Skeletal System {continued). 



Remove the integument from the head and branchial region so as to expose 

 the skull and the visceral arches. There are seven visceral arches, forming 

 the supporting framework of the branchial region (pharynx). The first is the 

 mandibular, represented by the upper and lower jaws, and the second is the 

 hyoid, which is divided into an upper or hyomandibular portion, attached 

 to the skull and serving (as the suspensorium) to connect the jaws with the 

 craniimi, and a lower portion, the hyoid proper or cerato-hyal. The other five 

 are the branchial arches, and each is divided into four segments. 



Aud. cp. 



Olf. 



r. a. S. 



L.). 



C. hy. 



Fi(, 



60. — Skull and Visceral Arches of the Dogfish (Scyllium) as seen from the left side. 

 After W. K. Parker. 



Or,, Cranium ; Olf. op., Olfactory capsule ; Aud. cp., Auditoi-y capsule ; Up. j., Upper jaw or palato-pterygoid 

 cartilage; L. j.. Lower jaw (Meckel's cartilage); Lg., Ligament; Hym., Hyomandibular; C. hy., Cerato- 

 hyal or hyoid proper ; Br. a. 1 to Br. a. 5, Branchial arches. 



F. The Nervous System. 



Slice away the cartilaginous cranium so as to expose the brain, taking care 

 to leave the eye and the auditorj"- organ (otocyst) of the left side uninjured. 

 Note the olfactory capsules lying over each nostril. 



1. The Brain. 



(a) The olfactory lobes lie close to the olfactory capsules, and are connected 

 to the cerebral hemispheres, each one by a long stalk. 



{b) The cerebral hemispheres (prosencephalon) are fused together to form 

 the anterior part of the brain. 



(c) The thalamencephalon is the insunk part behind the cerebral hemi- 

 spheres. It has a thin roof with a. choroid plexus of blood-vessels. 

 Its narrow cavity is the third ventricle, and its thick side-walls 

 are the optic thalami. From the posterior part of its roof a 

 slender stalk-like outgrowth, the pineal organ or epiphysis, projects 

 forward. An outgrowth from its floor is the infundibulum 



