Anatomy & Physiology
Holothuria hilla has a flexible body wall which contains longitudinal and circular muscles. This animal has a reduced endoskeleton made of individual particles of calcite called ossicles. Tables and buttons are present in the body wall (Dabbagh & Kamrani 2011) (Figure 4a). The presence of C-shaped ossicles in the longitudinal muscles classifies H. hilla in the subgenus Mertensiothuria (Samyn & Massin 2003).
Figure 4a. Illustrations showing the three types of ossicles found within the body wall of H. Hilla. A: buttons; B: c-shaped; C: tables. (Adapted from Samyn & Massin 2003; Dabbagh & Kamrani 2011)
When threatened, H. hilla may use its ossicles to harden its external surface, making it shorter and more rigid, and likely less desirable for predators to consume (Figure 4b). Unlike other Holothuriidae family members, this species do not have cuvierian tubules. Instead, it has the ability to eviscerate to distract predators.
Figure 4b. Picture showing the two different states of H. hilla. A: H. hilla in defensive state; B: H. hilla in relaxed state. (Note the difference distance between the papillae in the two different states)
Nervous System & Internal transport
Head and true brain for general coordination are absent in H. hilla (Preston 1993). They have a ring of neural tissue which encircles the oral end and five radial nerve cords that runs in the inner connective layer along its longitudinal axis of the body (Mashanov et al 2009). Each radial nerve cord consists of the inner hyponeural cord and outer ectoneural cord. When compared to ectoneural cord, the hyponeural cord is considerably thinner. The circumoral nerve ring is formed by the fusion of adjacent ectoneural band (Figure 4c).
Figure 4c. Simplified illustrations showing the nervous system of H. hilla. A: Section of lateral view of H. hilla (oral end). B: Cross section at the middle section. C: Magnified version in the boxed area of (B). (Adapted from Manshanov et al 2009)
H. hilla has an extensive network of haemal tissues (rete mirabile) which lines the intestines. This network is responsible for dispersing metabolites and nutrients to the rest of the body (Preston 1993). The presence of a water vascular system in H. hilla provides hydraulic pressure to enable movement of its tube feet. This system is filled with coelomic fluid and is not connected to the external environment. Coelomocytes that moves within the coelom have similar roles to blood cells in vertebrates. |