In this article we will discuss about the structure of Sea Pen (Pennatula) with the help of diagrams.
1. It is commonly known as “sea pen” and is found in the eastern coasts of North America.
2. The colour is usually red and the fully formed colony measures about 10 cm in height.
3. The colony is elongated, dimomorphic, feather-like and is differentiated into a lower peduncle or stalk and an upper rachis.
4. The peduncle is dialated at its lower tip into an end bulb, which remains burried in mud or sand at the sea bottom and is devoid of zoids.
5. The rachis is narow at two ends, dialated in the middle and bears two rows of lateral branches—the pinnules.
6. Each pinnule is a long, slightly curved, flattened and fleshy projection of the rachis and bears, along its upper margin, a row of anthocodia—the autozoids.
7. Each anthocodia (autozold) bears a ring of 8 tentacles, few gonads and mesentaries In multiples of eight (8). They serve to feed the colony and undertake the task of reproduction also.
8. The rachis, all through its length along dorsal and lateral sides, bears minute spinules— the siphonozoids.
9. The siphonozoids are without tentacles and gonads, with reduced mesentaries and are having distinct siphonoglyphs, which help in producing water currents.
10. Skeleton is a horny axis which supports only the peduncle and rachis but does not extend into pinnules.
11. Sexes are separate.