By: Javier Ovalles
Introduction to Annelida
Annelida, also known as Rengeroworms, include over 17,000 modern species including peanutworms, earthworms and leeches. Annelids are all bilaterally symmetrical animals. They range in size from much less than 1 mm in length to more than 3 m. The fundamental characteristic of the phylum is the division of the body into a linear series of cylindrical segments. he annelid body consists of a head region; a trunk, made up of metameres; and an unsegmented terminal region called the pygidium. Reproduction for the renegeroworms are sexual or asexual. Asexual reproduction is by fragmentation, budding, or fission. Among sexually reproducing annelids hermaphrodites are common, but most species have separate sexes. Fertilized eggs of marine annelids usually develop into free-swimming larvae. Eggs of terrestrial forms are enclosed in cocoons and hatch as miniature versions of the adults.
Earth Worms (Lumbricina)
Mating occurs on the surface, most often at night. Earthworms are hermaphrodites, they have both male and female sexual organs. The sexual organs are located in segments 9 to 15. Earthworms have one or two pairs of testes contained within sacs. The two or four pairs of seminal vesicles produce, store and release the sperm via the male pores. Ovaries and oviducts in segment 13 release eggs via female pores on segment 14, while sperm is expelled from segment 15. One or more pairs of spermathecae are present in segments 9 and 10 which are internal sacs that receive and store sperm from the other worm during copulation. As a result, segment 15 of one worm exudes sperm into segments 9 and 10 with its storage vesicles of its mate. Copulation and reproduction are separate processes in earthworms. The mating pair overlap front ends ventrally and each exchanges sperm with the other. The clitellum becomes very reddish to pinkish in color. Some time after copulation, long after the worms have separated, the clitellum (behind the spermathecae) secretes material which forms a ring around the worm. The worm then backs out of the ring, and as it does so, it injects its own eggs and the other worm's sperm into it. As the worm slips out of the ring, the ends of the cocoon seal to form a vaguely lemon-shaped cocoon in which the embryonic worms develop. They emerge as small, but fully formed earthworms, but lack their sex structures, which develop in about 60 to 90 days. They attain full size in about one year.
Leeches (Hirudinea)
Leeches are hermaphrodites meaning that a single individual is both male and female at the same time. Reproduction occurs through the production of cocoons that are either attached to a substrate where they develop or in the family Glossiphoniidae many species of leeches have the cocoons attached directly to the ventral surface of the parent. This allows the parent to protect and care for the young as they develop. This includes providing food for the young leeches after they hatch. The reproductive systems vary greatly between the different families of leeches. Some families, including the Glossiphoniidae, have a simple pair of gonopores that open to the ventral surface of the leech. Other families, such as the Richardsonianidae, have a complex system with a penis and vagina. All leeches have paired testisacs that are located within the midbody of the adult leech. The number of testisacs varies depending on the species of leech. The ovisacs of leeches are paired (only one pair) and can be long and tubular, or short and spherical in shape again depending on the family of leech. Leeches have internal fertilization with most mating by implanting a "spermatophore" (sperm package) into their partner. This implantation can by done through the use of a protrusible penis or by the hypodermic implantation of the spermatophore into the body of the partner. The sperm are then transported to the eggs where fertilization occurs.
Peanut Worms (Sipuncula)
Both asexual and sexual reproduction can be found in Sipunculans, although asexual reproduction is uncommon. Sipunculans reproduce asexually via transverse fission followed by regeneration of vital body components. Most sipunculan species are dioecious. Their gametes are produced in the coelomic lining, where they are released into the coelom to mature. These gametes are then picked up by the metanephridia system and released into the aquatic environment, where fertilisation takes place. Although some species hatch directly into the adult form, many have a trochophore larva, which metamorphoses into the adult after anything from a day to a month, depending on species. In a few species, the trochophore does not develop directly into the adult, but into an intermediate pelagosphaera stage, that possesses a greatly enlarged metatroch (ciliated band).