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).
Listed, below, are the eight cranial bones. Ethmoid: This bone forms the lower part of the eye orbits, and the ceiling of the nasal cavity. Frontal: This bone forms part of the cranial cavity, including the forehead, brow ridges and nasal cavity. Occipital: This bone forms the posterior and inferior part of the cranium.