Section 1. What is the anatomical orientation of the chondrocranium and brain?
One odd thing you may have noticed about sharks is that their skeletal elements are referred to as cartilages rather than bones. This is because over the course of their evolution, sharks have lost nearly all of their bony tissue and, in its place, they've evolved specialized types of flexible and resilient cartilage (Dean & Summers 2006). This includes the skull, which is made up of various cartilages split into two groups: the chondrocranium and the splanchnocranium.
The chondrocranium (meaning, "cartilaginous skull"), also known as the braincase, encases the brain whereas the splanchnocranium, also known as the viscerocranium, includes all the cartilages derived from the gill arches (all the other cartilages of the skull). The names "splanchnocranium" and "viscerocranium" come from the association between the gill arches and the gut: "viscero-" and "splancho-" both mean intestines in Latin and Greek, respectively. You'll start building with the chondrocranium, since this forms a foundation for the skull.
Orientation of the braincase and brain
In your kit, find the two halves of the chondrocranium and the brain (see the image below).
If the two halves of the chondrocranium are not already snapped together as a whole piece, how do they fit together? If you get stuck, open the hint below. When you're ready to check your work, open the Assess box below.
HINT: Find the flat surface
Find the completely flat surface of each half; this is where the two halves fit together.
ASSESS: Braincase made whole
The two halves of the chondrocranium, with the help of embedded magnets, should snap together like in the image below.
Now that you've solved how the two halves fit together, which end is rostral (toward the shark's snout or front end) and which end is caudal (toward the tail or back end)? Which side is dorsal (the shark's back) and which side is ventral (the shark's belly)?
Fill in the blanks on page 1 of your Notebook with these directional terms. When you think you've got it correct, use the following three hints to check your work.
HINT: Where are the nose and eye?
The arrows below show the positions of the nasal capsule (which houses the olfactory organ) and orbit (which houses the eye). Which do you think is which? Do you need to change any of your answers?
HINT: Nose in front
The labeled arrows in the image below indicate the nasal capsule (more rostral) and the orbit (more posterior). Do you need to change any of your answers?
