HEMICHORDATES: ARE THEY CHORDATES? MODULE 2

MODULE 2 : ARE THEY CHORDATES?

Unit 1 Hemichordates Most zoologists agree that the evolutionary ties between the chordates and hemichordates are closer than those between echinoderms and either phylum. The hemichordates were formerly considered a subphylum of the chordates, based on their possession of gill slits and a rudimentary notochord. It is now known that a notochord is lacking and what is present is a stomocord, which is a forwardly directed diverticulum of the gut, issuing from the buccal cavity. The gill slits in the pharynx are used primarily for filter feeding and only secondarily for breathing and are thus comparable to those in the protochordates. A tubular dorsal nerve cord in the collar zone may represent an early stage of the condition in chordates; a diffused set of nerve cells is similar to the uncentralised subepithelial plexus of echinoderms.

Classification Class Enteropneusta (Gr. entero, intestine + pneustikos, for breathing) Examples: Balanoglossus, Saccoglossus, Ptychodera, etc. Class (Gr. pteron, wing or feathers + branchia, gills). Order Cephalodiscida, example: Order Rhabdopleurida, example:

Figure 1: (Saccoglossus) Source: Google images

Unit 2

Biology of Balanoglossus Balanoglossus is a common genus of the class Enteropneusta. Members of the class, commonly called acorn worms, are marine worms which range in size between 10 and 40cm, although some can be as long as 2m. They are sluggish, worm-like that live in burrows or under stones, usually in mud or sand flats of intertidal zones.

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HEMICHORDATES: ARE THEY CHORDATES? MODULE 2

The body of Balanoglossus is soft, cylindrical with a ciliated and mucus-covered surface. The body is divided into a short conical (or tongue-like) proboscis, a short collar and a long trunk. The proboscis probes about in the mud and collects food in mucous strands on its surface. These detritus and food particles are carried posteriorly and ventrally to the groove at the edge of the collar and into the mouth before being swallowed. Large particles can be rejected by pulling the proboscis against the collar, thereby covering the mouth. The proboscis is also used to excavate. There is a coelomic sac (protocoel) in the posterior end of the proboscis into which extends the buccal diverticulum, a slender, blinding ending pouch of the gut that reaches forward into the buccal region. A slender canal connects the protocoel with a proboscis pore to the outside. By taking in water through the pores into the coelomic sacs, the proboscis and collar can be stiffened to aid burrowing. Contraction of the body musculature then forces the excess water Figure 2: External features of Balanoglossus (dorsal view) out through the gill slits, reducing the hydrostatic pressure and allowing the to move forward. The collar is short and muscular, cylinder-like, enclosing a pair of coelomic cavities (collar coelom) and opens by a pair of collar pores on the dorsal surface. The trunk is superficially ringed. It is divisible into anterior branchio-genital region, a middle hepatic region and a posterior abdominal region. A row of gill pores is located dorso-laterally on each side of the trunk just behind the collar. These open from a series of gill chambers that in turn connect with a series of gill slits in the sides of the pharynx. Cilia associated with the gill slits circulate water into the mouth and out of the body through pharyngeal gill slits. As water passes through the gill slits, gases are exchanged by diffusion between water and blood sinuses surrounding the pharynx. Respiratory gases and metabolic waste products (principally ammonia) can also be exchanged by diffusion through the body wall. Balanoglossus has a digestive tract which is a simple tube. Food particles caught are brought to the mouth by ciliary action, and directed along the ventral part of the pharynx and oesophagus to the intestine. Food is digested as diverticula of the gut, called hepatic sacs, release enzymes. The worm extends its posterior end out of the burrow during defaecation. At low tides, coils of faecal material, called castings lie in the substrate at burrow openings.

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HEMICHORDATES: ARE THEY CHORDATES? MODULE 2

The nervous system is ectodermal in origin and lies at the base of the ciliated epidermis. It consists of the dorsal and ventral nerve cords and a network of epidermal nerve cells, called a nerve plexus. Sensory receptors are unspecialized and widely distributed over the body. The circulatory system comprise a dorsal and ventral contractile vessel. Blood moves anteriorly in the dorsal vessel and posteriorly in the ventral vessel. All blood flowing anteriorly pass into a series of blood sinuses, called the glomerulus, at the base of the proboscis. The glomerulus also serves an excretory function. Sexes are separate (dioecious). Two rows of gonads are arranged dorso-laterally at the anterior region of the trunk. Each gonad opens separately to the outside. Fertilization is external. A ciliated larva, called the tornaria, swim in the plankton for several days to a few weeks. The larva settles to the substrate and gradually transform into the adult form.

Conclusion Hemichordates are commonly referred to as acorn worms and dwell in marine environments. Hemichordates do not possess the five principal features of chordates, hence they are not chordates. However, two of the five features of chordates can be observed in hemichordates and these are the dorsal nerve cord and the pharyngeal gill slit.

Assignment 1. Are hemichordates chordates? Explain. 2. List the three body regions of a typical hemichordate.

Reference Tropical Zoology (2013; 3rd Edition) by Ademola O. Segun

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