CHORDATE ZOOLOGY STUDY GUIDE - Zakharenko M.O. - 2015

CHAPTER 1. PHYLUM CHORDATA. LOWER CHORDATES

1.2. Subphylum Hemichordata

This subphylum is rarely classified separately. According to modern views, it is sometimes even grouped into a single phylum with all vertebrates. It comprises marine organisms whose Organization is intermediate between invertebrates and Chordates, largely reflecting their evolutionary status. That is, they exhibit features characteristic of both invertebrates and higher animals—chordates.

The first group of characteristics includes:

- a dermo-muscular sac with ciliated epithelium;

- two nerve cords (dorsal and ventral);

- Blood Vessels located on both the dorsal and ventral sides of the body.

Blood Circulation occurs counterclockwise;

- a developmental pattern identical to that of Echinoderms, involving a larval stage—the tornaria;

- a worm-like feeding method.

The second group of characteristics includes:

- a rudimentary notochord (stomochord);

- a well-developed dorsal nerve cord, which resembles a nerve tube in the anterior part of the body due to the presence of a cavity;

- a pharyngeal region of the digestive tract perforated by gill slits.

Enteropneusts (acorn worms) are distributed throughout all seas and oceans, while pterobranchs are found in the seas and oceans of the Southern Hemisphere. They inhabit various depths, from shallow waters down to 8000 m. Representatives of Enteropneusta settle in colonies on hard substrates of the sea floor. They feed on organic matter (dead organisms, small invertebrates, and plants), which they filter by passing sediment through their Esophagus. Pterobranchs obtain food using tentacles. These are dioecious animals that reproduce both sexually and asexually. Among them, viviparous species can be found (Xenopleura vivipara). Development mostly occurs through complex metamorphosis (via a tornaria larva). Some enteropneusts exhibit almost direct or intermediate development.

The first representative of hemichordates, Balanoglossus (Balanoglossus kowalevskii), was discovered in 1821 in the Pacific Ocean. In 1829, its larva was found in the Mediterranean Sea and was later described by the prominent zoologist I. I. Mechnikov in 1877. In 1866, A. O. Kovalevsky, studying the anatomy of Balanoglossus, discovered chordate characteristics in its Structure and pointed out the evolutionary relationship between this animal and vertebrates.

The body of Balanoglossus is elongated, covered with mucus, and vermiform (worm-like) in shape. It is divided into three regions: the proboscis, collar, and trunk (Fig. 9). A characteristic feature of hemichordates is the presence of an outgrowth in the middle of the proboscis, which is considered a rudimentary chord (notochord). Like the notochord, this outgrowth is formed from vacuolated Cells of endodermal origin and has a similar structure. Using the proboscis, which can extend and retract, Balanoglossus burrows through sand and mud. Together with the rudimentary notochord, the proboscis provides the animal with rigidity. Inside, it has a cavity connected to the first pair of gill slits. Due to this, the cavity opens to the outside and fills with seawater, which gives the animal additional turgor.

Balanoglossus lives in the soft mud or sand of coastal marine zones. As it burrows, it swallows large amounts of sediment, extracting PLANT AND ANIMAL organisms as well as Organic compounds for Nutrition.

Balanoglossus is characterized by a rudimentary coelom, an open Circulatory system, and bilateral body Symmetry (Fig. 9).

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Fig. 9. Structure of Balanoglossus:

A. General view: 1 - proboscis; 2 - proboscis cavity; 3 - Muscle; 4 - collar; 5 - proboscis pore; 6 - stomochord (hemichord);

7 - Mouth; 8 - gill slits; 9 - intestine; 10 - Heart; 11 - dorsal vessel; 12 - ventral vessel; 13 - dorsal nerve; 14 - ventral nerve; 15 - reproductive Organs. B. Longitudinal section through the anterior part of Balanoglossus: 1 - proboscis cavity; 2 - collar cavity; 3 - collar pore; 4 - stomochord (hemichord); 5 - mouth; 6 - gill slits; 7 - ventral vessel; 3 - dorsal vessel; 9 - Pericardium (heart sac); 10 - dorsal nerve cord; 11 - ventral nerve

The digestive organs consist of a mouth located on the ventral side of the Base of the proboscis, and a short Pharynx running along the collar, which transitions into the esophagus. The pharynx has numerous gill pores opening to the outside, which serve as respiratory organs. In the inner lower part of the pharynx, there is an endostyle (groove) containing glandular cells and ciliated epithelium. Tiny food particles suspended in the Water are trapped by the groove, bound by mucus, separated from the water by cilia, and directed into the esophagus and then into the intestine. Water passes through the mouth into the branchial region, where oxygenation of the blood occurs, and then exits through the gill pores. Due to this method of Respiration, these animals are called Enteropneusta (gut-breathers).

Posterior to the branchial region of the gut lies the hepatic region, where food is digested. The food then passes into the midgut, where absorption takes place, and finally into the hindgut, which terminates in the anus.

The circulatory system is open, consisting of ventral and dorsal longitudinal vessels and a system of lacunae. Contracting rhythmically, The Heart vesicle presses against the central sinus, forcing blood into the peripharyngeal vessels, which form branchial lacunae near the gill vessels; here, the blood is oxygenated before entering the ventral vessel. From the ventral vessel, blood is distributed to all Internal Organs and then reaches the dorsal vessel via the perienteric lacunae. Subsequently, due to negative pressure, the blood enters the central sinus. This circulation is maintained by a system of Valves. Such a direction of blood flow is not characteristic of chordates.

The excretory system is represented by various structures. First, there is the glomerulus, located near the notochord rudiment above the central sinus. Diffusion of fluids occurs between the central sinus and the glomerulus; Metabolic waste products pass through the glomerulus into the proboscis coelom and are excreted through the proboscis pore. Other excretory organs are nephridial tubes—coelomoducts—which open into the first pair of gill slits. Typically, there is only one, left, nephridium in the proboscis, and two in the collar.

The Nervous System of hemichordates, like that of echinoderms, consists of dorsal and ventral nerve cords. They are connected by a collar nerve ring and form a nerve tube. The nervous system of hemichordates is intermediate between that of invertebrates (indicated by the presence of a ventral cord) and chordates (characterized by a more developed dorsal cord).

There are no specialized Sensory Organs, but light-sensitive cells are located beneath the single-layered epithelium.

They reproduce sexually. Balanoglossus, like other enteropneusts, is a dioecious animal. Externally, males and females are indistinguishable. The Reproductive System of Balanoglossus is very simple. It consists of 30 pairs of Gonads in the form of oval sacs attached to the intestinal walls. Short ducts extend from the gonads, through which Gametes—eggs—are released into the water. Typically, the eggs attach to the walls of underground burrows, where Fertilization takes place. After some time, the egg hatches into a larva—the tornaria (which closely resembles the larva of echinoderms)—which features shimmering ciliated bands forming a corona, and an apical ganglion with a tuft of cilia. Its body is transparent. During the early Selection/3.html">Stages of development, it swims actively in the water. It grows mainly in length.

The described structural characteristics of this representative of hemichordates indicate that this subphylum occupies an intermediate position between invertebrates and chordates, sharing common ancestors with both. Hemichordates are of great evolutionary significance because they point to the relationship between chordates and other phyla, particularly echinoderms. They serve as a prototype of chordates, rather than their direct ancestors.