Mollusks
The Mullusca phylum is the largest among the invertebrates, and the largest marine phylum. It includes twenty-three percent of marine animals. It is not a very complex or advanced group of animals, but it is very interesting to learn about. This phylum includes animals such as the snail, clam, squid, octopus, and cuttlefish. The basic reproduction system in mollusks relies on external fertilization. All mollusks produce eggs, which usually emerge through the trochophore in adult females. However, some mollusks will occasionally use a process of internal fertilization. This process does not require more than one mollusk. This is called a hermaphrodite and one mollusk functions as both sexes. This requires a more complex reproduction system than that of external fertilization.