Section through a female medusiform sea daisy

They range in size from 0.03 to 0.54 in (0.9-13.6 mm) in diameter, including the marginal spines. They are translucent, with underlying skeletal structure and gonads visible.

Sea daisies have an outer water vascular ring system with lateral canals, ampullae and podia, not arranged in five radiating arms. An inner ring was originally thought to be also part of the water vascular system as it contained coelomocytes similar to those in the water vascular system but it may be part of the haemal system; however, this is not yet proven. The reproductive system is unique amongst echinoderms. The sexes are separate, and the gonoducts of each pair of 10 gonads are fused to form a single, interradial duct leading to a gonopore that opens in the females at the body margin, and in the males, slightly beyond the margin as the penial projection, which is most likely used for copulation. The highly filiform nature of the spermatozooa also suggests that these extremely modified cells do not contact the external environment, and that copulation takes place.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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