Internal Anatomy

Digestive System

The mouth opens with a muscular pharynx, which is anteriorly surrounded by a ring of ten calcareous plates (ossicles). In this species, the pharynx opens directly into the intestine (Ruppert, et al 2004). The intestine is relatively long once stretched out, but it remains coiled roughly three times the length of the sea cucumbers body before finishing in a cloacal chamber. The cloacal chamber is suspended by cloacal dilator muscles (Gmelin 1791).

Figure 11: Close up of Holothuria impatiens intestine

Nervous system

All holothurians have no true brain. H. impatiens possess a ring of neural tissue, which delivers nerves to the tentacles and the pharynx (Ruppert, et al 2004). This ring surrounds the oral cavity. Studies have shown that holothurians are able to function despite having their nerve ring removed (Gmelin 1791). This suggests that it does not play a fundamental role in nervous coordination. In conjunction with this, beneath each of the five ambulacra a nerve runs from the nerve ring to the anus.

The majority of holothurians do not have distinct sensory organs, however they do possess numerous nerve endings that are dispersed throughout the dermis, which allows the animal to have a sense of touch as well as a sensitivity to light (Kerr 2003).

Respiratory system

In order to extract oxygen from the water, sea cucumbers use respiratory trees. These trees are unique to holothurians situated at the posterior end of the body. They are made up of a sequence of thin tubules that extend from the same duct, and are located on either side of the digestive tract. Gas exchange takes place across the thin walls of the tubules (Ruppert, et al 2004).

The respiratory trees are attached to and ventilated by the cloaca inside the anus, so as water is drawn in, it passes through the anus, the oxygen content is extracted and the remaining water is then expelled back out the anus. The intestine is not the only excretory organ within the sea cucumber; the respiratory trees also do their part by excreting nitrogenous waste through means of diffusion across the tubule walls in the form of ammonia (Gmelin 1791).


Figure 12: Close up of Holothuria impatiens respiratory trees

Circulatory systems

The water vascular system (WVS) is what controls sea cucumber locomotion, once the hydraulic pressure extends the tube foot the animal can begin to move. The WVS is filled with coelomic fluid, which also bathes the organs directly. Similar to white blood cells in vertebrates, phagocytic coelomocytes are created inside the haemal vessels and move throughout the circulatory system and the body cavity (Ruppert, et al 2004).


Figure 13: Dissection of Holothuria impatiens, labelling key features of the internal anatomy