Which of the following do NOT have a mesoderm?
A. Jellyfishes
B. Earthworms
C. Flatworms
D. Octopi
E. Snails
To determine which of the given organisms do not have a mesoderm, we need to understand what a mesoderm is. The mesoderm is one of the three primary germ layers in early embryo development. It gives rise to various structures such as muscles, bones, circulatory system, and connective tissues.
To answer the question, we can examine each organism and identify whether they possess a mesoderm:
A. Jellyfishes: Jellyfishes are part of the phylum Cnidaria and belong to a group called Diploblasts. Diploblasts have two germ layers (endoderm and ectoderm) and lack a true mesoderm. So, jellyfishes do not have a mesoderm.
B. Earthworms: Earthworms are examples of Annelids and belong to a group called Triploblasts. Triploblasts have all three germ layers (endoderm, mesoderm, and ectoderm). Therefore, earthworms do have a mesoderm.
C. Flatworms: Flatworms, also known as Platyhelminthes, are Triploblasts and possess all three germ layers, including the mesoderm. So, flatworms do have a mesoderm.
D. Octopi: Octopi are part of the phylum Mollusca. Although mollusks are Triploblasts, not all mollusks possess a true mesoderm. In the case of octopi, they do have a mesoderm, which gives rise to their muscle and organ tissues. Therefore, octopi have a mesoderm.
E. Snails: Snails are also mollusks, and like octopi, they have a true mesoderm. The mesoderm in snails develops into muscles, reproductive organs, and other structures. Hence, snails have a mesoderm.
In conclusion, the organism that does NOT have a mesoderm is A. Jellyfishes.