ZOOLOGY OF CHORDATES: STUDY GUIDE - Zakharenko M.O. - 2015

CHAPTER 1. PHYLUM CHORDATA. LOWER CHORDATES

1.1. Subphylum Acrania (Cephalochordata)

The Acrania represent the most advanced group of lower Chordates, displaying the principal CHARACTERISTICS OF THE entire phylum to the greatest degree. They occupy an intermediate evolutionary position between lower chordates and vertebrates. The most Characteristic Features of this subphylum are:

- complete body segmentation, with no distinct HEAD region;

- the notochord extends into both the head and trunk regions;

- the notochord is significantly longer than the neural tube;

- absence of a cranium in the anterior part of the body;

- absence of a Vertebral Column surrounding the notochord;

- presence of an atrial (peribranchial) cavity;

- closed Circulatory system, with no Heart;

- absence of paired limbs;

- single-layered epidermis;

- nephridial excretory system;

- striated musculature represented by myomeres.

Currently, about 30 species of this subphylum are known, grouped into a single Class comprising one order and three families.

Subphylum Acrania

Class CEPHALOCHORDATA

Order AMPHIOXIFORMES

Family BRANCHIOSTOMIDAE

These animals possess a bilaterally symmetrical body Structure. There are approximately 20 species of typical lancelets, among which the most prominent representatives are the European lancelet (Branchiostoma lanceolatum) and the Asian lancelet (B. belcheri).

General characteristics of acranians using the lancelet as an example.

The lancelet is a translucent marine animal up to 8 cm in length (Fig. 2).

Fig. 2. External Anatomy of the lancelet:

1 - gill slits; 2 - tentacles; 3 - oral hood; 4 - nerve cord; 5 - notochord;

6 - caudal fin; 7 - anus; 8 - atriopore; 9 - ventrolateral folds

It is most commonly found in sandy seabeds at depths of up to 30 m, where it burrows into the substrate, leaving only the anterior part of its body exposed. The lancelet gets its name from the distinctive shape of its tail region, which resembles a lancet. The body is laterally compressed. A small fold, the dorsal fin, runs along the back and frames the tail region, forming a lanceolate caudal fin. At the anterior end of the body, on its ventral side, is the oral hood bearing 10-20 pairs of tentacles, at the bottom of which lies the Mouth.

The integument of the lancelet, as in all chordates, consists of two layers: the epidermal (outer) layer and the corium (inner layer). The epidermis is single-layered and externally covered with a thin cuticle formed by the secretions of epidermal glands, which protects the delicate Skin from damage. The corium is represented by a thin layer of gelatinous Connective Tissue. The notochord serves as the Axial Skeleton, running from the anterior to the posterior end of the body, and is referred to in the lancelet as the notochord (Figs. 3, 4).

Fig. 3. Cytology/practical/54.html">Longitudinal section of the lancelet:

1 - notochord; 2 - nerve cord; 3 - Oral Cavity; 4 - gill slits in the Pharynx; 5 - peribranchial cavity;

6 - atriopore; 7 - hepatic caecum; 8 - intestine; 9 - anus

The musculature consists of Muscle segments, or myomeres, separated by connective tissue partitions known as myosepta.

Feeding and Respiration. Feeding is passive. The mouth is located at the Base of the oral hood, which is surrounded by tentacles. The pharynx is large and perforated by numerous gill slits (about 100) that open into the peribranchial cavity, which has an opening called the atriopore (Fig. 4).

Fig. 4. Transverse section of the lancelet

A - at the level of the pharyngeal region; B - at the level of the midgut: 1 - nerve cord; 2 - myomere (musculature); 3 - roots of the dorsal aorta;

4 - gonad (Ovary); 5 - hypobranchial groove (endostyle); 6 - ventral aorta; 7 - metapleural folds;

8 - peribranchial (atrial) cavity; 9 - gill slits; 10 - nephridia; 11 - coelom; 12 - ventral nerve; 13 - dorsal nerve;

14 - notochord; 15 - subintestinal vein; 16 - dorsal aorta; 17 - dorsal fin

The gill slits are separated from one another by thin interbranchial septa covered with ciliated epithelium. The movement of the tentacles generates a Water current that flows from the pharynx through the gill slits into the peribranchial cavity and out into the external environment through the atriopore. Running along the bottom of the pharynx is a groove lined with glandular and ciliated Cells called the endostyle; its function is to envelop the food particles—which enter the pharynx with water—in secreted mucus and direct the food masses into the intestine. Narrowing sharply at the posterior end, the pharynx transitions into a short intestine. The intestine is straight and undivided into distinct regions.

A blind hepatic caecum, homologous to the Liver of vertebrates, extends from the ventral side of The Stomach and begins immediately posterior to the pharynx. Food Digestion takes place both within the lumen of the hepatic caecum and throughout the entire intestine. Undigested residues are eliminated through the anus.

The CIRCULATORY SYSTEM OF the lancelet is closed and consists of a series of vessels carrying colorless Blood. A heart is absent; blood flow is maintained by the pulsating ventral aorta and the afferent branchial Arteries, or "branchial hearts." There is a single circulatory loop (Fig. 5).

Fig. 5. Circulatory System of the lancelet

1 - ventral aorta; 2 - branchial arteries; 3 - roots of the aorta; 4 - carotid arteries; 5 - dorsal aorta;

6 - anterior cardinal Veins; 7 - posterior cardinal veins; 8 - ducts of Cuvier; 9 - venous sinus;

10 - subintestinal vein; 11 - hepatic portal system; 12 - hepatic vein; 13 - caudal vein

The excretory system is represented by numerous (up to 100 pairs) nephridia located in the pharyngeal region (Fig. 6).

Fig. 6. Excretory system of amphioxus

A - entire tubule with nephrostomes and solenocytes: 1 - upper end of the gill slit; 2 - opening of the renal tubule into the atrial cavity; 3 - nephrostomes; B - part of the excretory tubule wall with attached solenocytes

The Central Nervous system is represented by a thick-walled neural tube lying above the notochord, with its anterior end not reaching the tip of the notochord (hence the name of the subphylum "Cephalochordata").

Sense Organs are simple: photosensitive structures are represented by a small unpaired pigment spot in the anterior region of the neural tube and Hesse eyespots located along the edges of the neurocoel throughout the entire neural tube; mechanical stimuli are perceived by nerve endings in the superficial skin layer and oral tentacles, where chemoreceptors detecting chemical stimuli are also located. At the anterior end of the body, there is Kölliker's pit, which Functions as an Olfactory Organ.

Reproductive System, reproduction, and development. Amphioxi are dioecious. The Gonads (about 25 pairs) are metamerically arranged rounded swellings on the body wall connected to the atrial cavity. Acraniates lack specialized duct systems for gamete release. Sexual maturity is reached in the 2nd to 3rd year of life. Mature Gametes are released into the peribranchial cavity via rupture of the gonad wall, and subsequently pass outward through the atriopore (Fig. 7).

Fig. 7. Embryonic development of amphioxus. A - longitudinal sections. I - blastula. II-IV - Gastrulation. V-VI - Formation of the mesoderm,

notochord, and nervous system. 1 - animal pole and 2 - vegetal pole; 3 - archenteron (gastrocoel); 4 - blastopore; 5 - neural canal;

6 - neurenteric canal; 7 - neuropore; 8 - mesodermal fold; 9 - coelomic pouches; 10 - notochord; 11 - future mouth; 12 - future anus. B - transverse sections: 1 - ectoderm; 2 - endoderm; 3 - mesoderm; 4 - gut cavity; 5 - neural plate;

6 - neural tube; 7 - neurocoel; 8 - notochord; 9 - coelom (secondary body cavity)

The larvae live in the water for about three months, moving by means of epidermal cilia and later by tail movements. The larva feeds actively before settling to the bottom. As a result of metamorphosis, an oral hood with a "ring" of tentacles and all definitive organs are formed around the mouth (Fig. 8).

Fig. 8. Transverse section of an amphioxus larva:

1 - notochord; 2 - secondary body cavity (coelom); 3 - gonotome; 4 - gut; 5 - myotome; 6 - cutaneous layer; 7 - sclerotome; 8 - neural tube

Family EPIGONICHTIDAE. It includes 6 species, whose main structural feature is Asymmetry. The principal representatives are Epigonichthys and Asymmetron.

Family AMPHIOXIDIDAE. These animals are characterized by a neotenic form, are deep-sea dwellers, and are very small (no more than 16 mm). The representative genus is Amphioxides.

Review Questions

1. What characteristic features define the members of the phylum Chordata?

2. What is the notochord? What are its functions?

3. What respiratory organs are characteristic of aquatic and terrestrial chordates?

4. Which animals are primarily aquatic and which are secondarily aquatic?

5. What adaptations does amphioxus possess for a sedentary lifestyle?

6. What are the external morphological features of amphioxus?

7. How do feeding and gas exchange occur in amphioxi?

8. What are the Structural Features of the circulatory system and Blood Circulation in amphioxus?

9. What do you know about the reproduction and development of amphioxus?

10. What is indicated by the presence of features in amphioxus that are characteristic of both vertebrates and invertebrates? Provide an argumentative answer.

11. THE ORIGIN OF chordates. Hypothetical ancestors among invertebrates.

12. General characteristics of chordates and their division into subphyla based on the level of development and structural features.

13. Subphylum Acrania as the ancestors of chordates. Progressive and primitive features of amphioxuses. Systematics of the subphylum.

14. What are the Main characteristics of the Representatives of the subphylum Acrania?

15. What are Hesse's ocelli?

16. What is the endostyle?

17. How does the embryonic development of amphioxus proceed?

18. How does mesoderm formation occur in amphioxus?

19. What is The structure of a somite in amphioxus?

20. What is the STRUCTURE OF THE excretory system in amphioxus?

21. What is The Significance of acraniates in nature and human life?