As though it were not enough to have one opening to feed with, researchers at Friday Harbor Laboratories, Washington find that a sea cucumber Parastichopus californicus is able to use its anus¹ to access nutrients and energy materials carried or dissolved in seawater.  What happens is that as seawater is pumped into and out of the respiratory trees via the anus by rhythmic contractions of the rectum (see drawing above Right and photograph below Left), organic particles and dissolved nutrient-containing macromolecules are carried into the trees in the flow and taken up by the lining of the respiratory-tree lumen.  By use of ¹⁴C-labelled single-celled algae (Rhodomonas sp.) and iron-containing protein ferritin and polysaccharide dextran, the authors are able to follow paths of assimilated ¹⁴C-label and iron (ferritin) reaction product.  High concentrations are initially found within the cells lining the respiratory trees (see section of respiratory tree (below Right). After translocation to various organs the next highest concentrations are found within the rete mirabile² of the hemal system (see graph below, middle).  The function of this complex cluster of vessels, and the hemal system in general, has long puzzled biologists.  In some echinoderms parts of the hemal system appear to pump; in others, some vessels are open while others appear to be occluded.  In identifying³ the hemal system’s possible involvement with internal translocation of assimilated materials the authors have made a significant discovery that is sure to promote future research work.  Jaeckle & Strathmann 2012 Invert Biol 132 (1): 62.

NOTE¹  the authors term this a second functional mouth, but of course this is not true.  In deuterostomes such as echinoderms and chordates an opening, the blastopore, forms during gastrulation and becomes the anus.  A second opening, or deuterostome, develops later and becomes the mouth.  Mouths, by definition, open into the anterior ends of digestive tracts.  There is, therefore, only one mouth in a sea cucumber.   It is quite possible that any thin-skinned organ in a sea cucumber, such as the oral tentacles or tube-feet, could also be sites of uptake of dissolved organic matter

NOTE²  in no other echinoderm is this complex system of vessels so well-developed as in a sea cucumber

NOTE³ another possibility, not specifically mentioned by the authors, is that the particulate and dissolved materials absorbed by the cells lining the respiratory-tree lumen may and translocated to the rete mirabile may be simply a process of dealing with unwanted contamination.  One function of these vessels, not yet demonstrated convincingly in experiments, is thought to be excretory