Anatomy & Physiology

A dissection was conducted in conjunction with Ellen Jackson on Echinaster luzonicus, a sea star of similar morphological characteristics. Photos from the dissection are directly from Ellen’s dissection, with her permission, to assist in visual explanations of L. multifora anatomy. The radial canal acts as the skeleton of sea stars and is made up entirely of calcium carbonate (Ruppert et al. 2004, p. 373). Within this radial canal are connected ampullae that are effectively where the tube foot meets the body. At the opposite end of the tube foot is the sucker where ocelli are located. The ocelli are the ‘simple eye’ in these echinoderms (Ruppert et al. 2004, p. 373). 



Figure 1 (above left) and Figure 2 (above right) from the dissection, showing the skeletal radial canal and the ocelli respectively. 


All asteroids have pentamerous symmetry, which is a type of radial symmetry. This is so named because the body can be divided equally into 5 parts around an axis in the middle (Ruppert, 2004, p. 873). Many aspects of the anatomy of Linckia multifora are uniform across all echinoderms, with only a few specific differences mentioned for the Order, Valvatida. 

The water vascular system (WVS) travels through the entire body of the sea star, including each arm. It is lined with muscles for tube feet movement, and cilia for the flow of internal transport functions (Ruppert, 2004, p. 879). It is composed of internal coelomic canals and is not restricted internally. The WVS extends out of the body in the form of tube feet. The WVS in asteroids is very well developed and is required for gas exchange, adhesion (with the tube feet), locomotion and prey manipulation during feeding (Ruppert, 2004, p. 879).

Mutable, or catch, connective tissue allows the body to change the rigidness of its dermis skin layer (Motokawa, 1991; Ruppert, 2004, p. 873). This change in flexibility does not occur under stress, but is actively changed on a daily basis, particularly for the purpose of feeding. The mechanics of the tissue properties can change rapidly under the control of the nerves, and not the muscles (Motokawa, 1991). Studies have shown, using Linckia species that the stiffness of an asteroid body can potentially change by two orders of magnitude (Motokawa, 1991).

Ossicles are a honeycomb-like structure that has the ability to increase strength, prevent fractures of the body, and reduce the weight of the organism. They come in a variety of shapes, including rods and spines that vary with each different species (Ruppert, 2004, p. 876). Ossicles have the ability to extend above the surface of the body (e.g. spines). The internal and external ossicles are covered by the epidermis as they are endoskeletal (Ruppert, 2004, p. 876). Pedicellariae are compound ossicles that move around and function on the surface of the body like forceps (Ruppert, 2004, p. 876). These pedicellariae are used for defence against smaller animals and settling larvae. Valvatida have unique sessile pedicellariae absent of stalks, but have still retained the two jaws that act like forceps for protection.