(a) to 1(d) and answer Q. No. 1(e) : (a) Canal system in sponges (10 marks) (b) Nematocysts of Aurelia (10 marks) (c)…

(a) Canal system in sponges

The canal system is the water-circulation network of sponges. Choanocyte flagella drive a one-way current that brings food and oxygen and carries wastes and gametes out. Three grades are recognised.

• Ascon: the ostia open straight into a large spongocoel lined with choanocytes, and water leaves through one osculum (Leucosolenia).
• Sycon: the body wall is folded; water passes from incurrent canals through prosopyles into radial canals lined with choanocytes, then through apopyles into the spongocoel and out of the osculum (Sycon, Grantia).
• Leucon: the spongocoel is lost; water passes through branching incurrent canals into many small flagellated chambers and leaves by excurrent canals to the osculum (Spongilla, Euspongia).

From ascon to leucon the choanocyte surface increases greatly without any increase in body size, so a larger sponge can filter more water. The canal system also serves respiration and excretion.

(b) Nematocysts of Aurelia

Nematocysts are stinging organelles produced inside cnidocytes, which are concentrated on the tentacles, oral arms and margin of Aurelia. Each is a capsule closed by an operculum and holding a coiled hollow thread, with a trigger hair (cnidocil) outside. Aurelia has three types: atrichous isorhizas (smooth thread, uniform width), holotrichous isorhizas (thread covered with spines) and heterotrichous microbasic euryteles (a short shaft with spines and a thinner thread).

On stimulation by touch or chemicals, water enters the capsule, the pressure rises sharply, the operculum opens and the thread everts explosively, like the finger of a glove turned inside out. The thread entangles or penetrates the prey and may inject toxin. Each nematocyst fires once and is replaced. They stun small plankton so that the oral arms can carry it to the mouth.

(c) Trochophore larva and its evolutionary significance

The trochophore is a free-swimming, top-shaped, ciliated larva of marine polychaetes (for example Nereis), many molluscs and several other spiralian phyla. It has an apical tuft of cilia, a prominent ring of cilia, the prototroch, around the equator, sometimes a postoral metatroch and a posterior telotroch, and a complete gut with mouth and anus. Protonephridia and a small blastocoel are present. The larva swims and feeds by ciliary currents that sweep particles to the mouth.

Significance: the similar trochophore of annelids and molluscs, with spiral cleavage, supports their common ancestry and the grouping of Annelida, Mollusca and allied phyla in the Spiralia (Lophotrochozoa). The larva of an annelid changes into a segmented worm by growth from a posterior zone, which suggests that segmentation arose later on a trochophore-like base. Haeckel's trochozoon and Nielsen's trochaea theories derive it from a ciliated ancestral adult. Larval likeness is evidence of relationship but not proof of any one ancestral form.

(d) Origin of tetrapods

Tetrapods arose from lobe-finned (sarcopterygian) fishes in the Devonian, about 370–390 million years ago. The ancestors had paired fins with bony axes (humerus, radius and ulna homologues), lungs and choanae. The steps are shown by fossils:

• Panderichthys and Tiktaalik (elpistostegids) had a flattened head, neck, ribs and a fin with wrist-like bones;
• Acanthostega and Ichthyostega were limbed tetrapods, but with 8 and 7 digits and still aquatic or amphibious, while Tulerpeton had 6; the five-digit limb came later;
• trackways from Zachełmie in Poland suggest limbed animals even earlier.

The move may have occurred in shallow, oxygen-poor, vegetation-choked freshwater, where limbs helped moving in the shallows and lungs helped breathing. Land living came later, and a gap in the early Carboniferous fossil record is known as Romer's gap.

(e) Avian brain (labelled diagram)

The diagram should show, in dorsal view, from the front:

• two small olfactory lobes at the front;
• large, smooth cerebral hemispheres, with no corpus callosum and no folds, the forebrain enlarged by nuclei (the Wulst and the dorsal ventricular ridge);
• a small pineal body between the hemispheres and the cerebellum;
• two large optic lobes on the sides of the midbrain, displaced laterally, from which the optic nerves run;
• a large median cerebellum with transverse folds (vermis) and small lateral flocculi, for flight coordination;
• the medulla oblongata, continuing into the spinal cord, with the cranial nerves.

On the ventral side are the optic chiasma, the infundibulum and the pituitary. The features that characterise the bird brain are the smooth cerebrum, the large optic lobes and the large cerebellum, which suit its sight and flight.