In open circulatory system, the blood (i.e. hemolymph) bathes the organs and tissues directly. Examples of animals with open circulatory system are arthropods and most mollusks. In closed circulatory system, the blood circulates within blood vessels throughout the body. Therefore, there is a distinction between blood and lymph. Examples of animals with closed circulatory system are annelids, few mollusks, and vertebrates.
The skull base is the caudal boundary of the cerebral cranium. Looking at it from the inside it can be subdivided into the anterior, middle and posterior cranial fossae. The skull base comprises parts of the frontal, ethmoid, sphenoid, occipital and temporal bones. The face is referred to as all skull bones fronto-caudally connected to the cerebral cranium. Prominent representatives are the maxilla (upper jaw) and the mandible (lower jaw).
A) The Neurocranium (Chondrocranium). The neurocranium is the portion of the skull that protects the brain and certain sense organs. In the Elasmobranchs (sharks and rays) it is composed of cartilage (chondrocranium), but in most other vertebrates, the cartilage is replaced by bone (endochondral or replacement bone).he dermatocranium forms most of the skull and functions as a protective shield for the brain. See the dermal armour of Amia, the bowfin, which sits on the cartilaginous neurocranium. In other animal groups the cartilage disappears and when you look down on the skull you are looking at the bones of the dermatocranium.