Four Specimens, One Misleading Character
A first-year batch surveys two sites - a rocky shore near Rameswaram and a campus pond - and brings back four unidentified animals. Their notebook reads:
Specimen P: sessile and colonial, encrusting rock. The body wall is perforated by many pores, water leaves through one large opening at the top, and no true tissues are visible under the microscope.
Specimen Q: a soft, unsegmented body with a muscular foot, a rasping radula and a mantle that secreted a shell. Dissection shows the main body cavity is a blood-filled haemocoel, with only a small coelomic sac around the heart.
Specimen R: five-armed, with a calcareous endoskeleton, hundreds of tube feet and a ring canal with radial canals. The adult is radially symmetrical; the larva collected in the plankton tow was bilaterally symmetrical.
Specimen S: a segmented worm with a clitellum, a closed circulatory system and a fluid-filled cavity divided by transverse septa.
Before submission, the students must defend every assignment with more than one feature. Their teacher warns that one specimen will be misclassified if they rely only on adult symmetry, and asks for a prediction about hydrostatic skeletons.
1. Assign each specimen to a phylum and give two structural features that exclude the other three phyla. Which specimen is a deuterostome, and on what basis would you defend that without watching its development?
Read the worked answer
P - Porifera. Ostia leading into a spongocoel with a single osculum, and choanocytes driving water flow. There are no true tissues (cellular level of organisation) and no organs, so P cannot be a cnidarian (tissue level, one opening serving both intake and output, stinging cells) or any triploblastic phylum.
Q - Mollusca. Muscular foot, radula, mantle secreting a shell, plus a reduced coelom with a haemocoel. The radula and mantle are unique to molluscs; there is no metameric segmentation, so it is not an annelid.
R - Echinodermata. A water-vascular system with tube feet and a calcareous endoskeleton of mesodermal ossicles. The water-vascular system is diagnostic; no other phylum here has it.
S - Annelida. Metameric segmentation with septa and a clitellum, plus a closed circulation.
Deuterostome: R. The markers are radial and indeterminate cleavage, enterocoely, and the blastopore giving rise to the anus. Echinoderms are the only invertebrate deuterostomes in this collection; P, Q and S are protostomes (spiral, determinate cleavage, schizocoely, blastopore giving the mouth). Because development was not observed, the practical evidence is the mesodermal skeleton of ossicles and the water-vascular system, both of which are deuterostome-style mesodermal derivatives.
2. A student groups Q with S because 'both are coelomate protostomes', and proposes placing R beside cnidarians because 'both are radially symmetrical'. Evaluate both moves.
Read the worked answer
Q with S: partly defensible, but far too coarse to justify a grouping. They share protostome characters - spiral determinate cleavage, schizocoely, and in many molluscs a trochophore-type larva. However the coeloms are not equivalent: the annelid has a true coelom lined by peritoneum and divided by septa, whereas in the mollusc the coelom is reduced to the pericardial and gonadal cavities and the main space is a haemocoel. Phylum assignment must rest on the diagnostic structures - radula and mantle for Q, segmentation and septa for S - not on two shared abstractions.
R with cnidarians: wrong. Cnidarians are diploblastic and at tissue level, with a gastrovascular cavity, no mesoderm and no organ systems; radial symmetry persists throughout life. R is triploblastic, organ-system level, has a coelom, mesodermal calcareous ossicles, a water-vascular system, and a bilaterally symmetrical larva. Its radial symmetry is secondary (pentaradial) and superimposed on a bilateral body plan.
The general lesson: adult symmetry is a single, easily misleading character; larval symmetry and embryonic origin of the coelom are stronger. R is the specimen the teacher meant.
3. Predict which specimens depend on a hydrostatic skeleton and which do not. Explain what the presence or absence of a true coelom has to do with your prediction.
Read the worked answer
S (annelid) - yes. A true coelom filled with incompressible fluid, enclosed by circular and longitudinal muscle layers and compartmentalised by septa, allows localised pressure changes, so contraction of circular muscle lengthens a segment while longitudinal contraction shortens and widens it. That is what makes peristalsis possible.
Q (mollusc) - largely no. The coelom is reduced, so there is no large fluid compartment to act as a skeleton; the shell provides support and protection, and the foot moves by muscular waves over mucus.
R (echinoderm) - no, though it uses hydraulics. The water-vascular system is a hydraulic system for tube feet (and the mutable collagenous tissue holds posture), but it is not a general hydrostatic skeleton for locomotion of the whole body.
P (sponge) - no. There is no coelom and no muscle; support comes from spicules and the skeleton, and movement is by water flow driven by choanocytes.
The principle you would state in the exam: a hydrostatic skeleton needs a fluid-filled cavity that is sealed or compartmentalised, surrounded by contractile tissue, with antagonistic muscle groups. Remove the cavity (P) or reduce it (Q), and support must come from a hard skeleton instead.