Ethenone is an organic compound made out of the elements Carbon, Hydrogen, and Oxygen. It is an example of a covalent molecule. The Hydrogens are single bonded to the carbon and then the carbons and oxygen are double-bonded to each other. Additionally, because there are 2 Hydrogens and 1 Oxygen the ratio is 2:1, as stated in the question.
<u>Mollusca:</u> Unsegmented exoskeleton, with no appendixes articulated to them. This skeleton has a calcareous origin.
<u>Arthropoda:</u> Segmented exoskeleton, with different appendixes articulated to them, which develop different functions. This skeleton has a chitinous origin.
Explanation:
Clams are bivalves. Their <u>calcareous</u> exoskeleton has two valves dorsally <u>articulated to each other</u>. They do not articulate with any appendix, although. These animals also have a <em>foot, a small head, and a developed mantle cavity</em>. The valves are convex, oval, and <u>unsegmented</u><u>.</u> They are composed of three layers: periostracum, prismatic, and pearly. En the dorsal portion of each valve, there is a bulge called the umbo, which is the oldest part of the valve. The articulation has some teeth-shaped structures that are intercalated, maintaining the valves together and fixed.
Grasshoppers have a <u>chitinous</u> exoskeletondivided into three principal <u>segments or regions</u> that allow their movement: head, thorax, and abdomen. The head is orientated in such a manner that their feeding apparatus is heading down. Also, the head segment has composed eyes and antennae. The thorax is the medial region of the body. It has three segments: the prothorax, mesothorax, and metathorax. Each of these segments articulates with a pair of legs. Finally, the abdomen is composed of many segments that might reach up to 7 or 11.