ZOOLOGY STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 12. GENERAL CHARACTERISTICS OF PHYLUM CHORDATA (Chordata)

2. General Characteristics of Subphylum Acrania. Class Cephalochordata

The Subphylum Acrania includes about 30 species of the most primitive Chordates.

General characteristics of the subphylum Acrania:

1. The notochord and nerve cord persist throughout their life.

2. The Skull and any rudiments of vertebrae are absent.

Підтип Безчерепні містить лише один клас Головохордові (Cephalochordata).

The most thoroughly studied cephalochordate is the lancelet, which is found in many seas, including the Black Sea. The name 'lancelet' comes from its resemblance to a lancet, a surgical instrument. The lancelet was first described in 1774 by the prominent scientist Peter Simon Pallas, who classified it as a mollusk. Later, the outstanding zoologist Alexander Onufrievich Kowalevsky proved that the lancelet is a member of the phylum Chordata.

Lancelets are marine animals 4-8 cm in length (Fig. 52). They inhabit temperate and warm seas on sandy shallows, burrowing into the sand and exposing only the anterior end of their body. Their lanceolate, pinkish body is laterally compressed and pointed at both ends. On the dorsal side, There is a fold of Skin forming a dorsal fin that merges into the caudal fin. Along the lower edge of the tail is the ventral fin.

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Fig. 52. Structure OF THE lancelet (after Naumov, Kartashev, 1979):

1 — anus; 2 — Gonads; 3 — atriopore; 4 — intestine; 5 — hepatic caecum;

6 — Pharynx with gill slits; 7 — atrium; 8 — tentacles; 9 — notochord; 10 — nerve cord.

The integument of the lancelet consists of a single-layered epithelium covered with a thin cuticle, and gelatinous Connective Tissue.

The internal Skeleton of the lancelet is represented by an elastic notochord that extends along the entire body and persists throughout its life.

The digestive tube is located beneath the notochord. It begins with an oral hood surrounded by tentacles. The movement of the tentacles surrounding the Mouth creates a Water current. Along with the water, unicellular plants and animals, as well as small crustaceans, enter the pharynx, and from there, pass into the undivided intestine. A hepatic caecum branches off from the ventral side of the anterior part of the intestine. The digestive tract opens externally through the anus. By its mode of feeding, the lancelet is a typical filter feeder.

The excretory Organs of the lancelet are represented by numerous (up to 90 pairs) tubules called nephridia, which are located along the dorsal part of the pharynx and open into the atrium, from where excretory products are discharged outward through the atriopore.

Respiration occurs simultaneously with feeding. There are about 100 pairs of gill slits in the walls of the pharynx, surrounded by capillaries of the Circulatory system in the pharyngeal Tissues. Oxygen from the water washing over the gill bars enters the Blood and is distributed throughout the body. To some extent, Gas Exchange in the lancelet can also occur through the body integument.

The circulatory system is closed, consisting of two main Blood Vessels (the ventral aorta and the dorsal aorta), from which numerous smaller vessels branch off (gill Arteries, carotid arteries, subintestinal vein, hepatic vein). Blood flow is driven by the pulsation of the walls of the ventral aorta. Thus, acranians have only a single circulatory circuit. The blood is colorless and contains no

respiratory pigments. The small size of the animals and their thin skin allow the blood to be oxygenated not only in the gill arteries but also in all superficial Vessels of the body.

The nerve cord, with nerves branching from it, is located above the notochord. Since the lancelet lacks a Brain, it also lacks a skull.

The Sensory Organs are poorly developed. Light is perceived by photosensitive Cells. There are cells on the animal's body that perceive either mechanical stimuli (tactile cells) or chemical stimuli (chemoreceptors). The olfactory pit (Kölliker's pit) is also lined with chemoreceptors.

The animals are dioecious (males and females). The gonads are paired (up to 25 pairs) and lack their own ducts. Gametes are released into the atrium, and from there, they pass outside through the atriopore. Lancelets reproduce only once in their lifetime, as the gametes are released through ruptures in the gonad walls, which do not regenerate. Fertilization is external.

Development is indirect: a larva develops from the egg in the water, swimming and feeding actively, and later settles to the bottom, metamorphosing into an adult animal that leads a passive lifestyle.

Structural Features of the lancelet as a primitive chordate:

✵ The notochord and nerve cord persist throughout life.

✵ The integument consists of a single-layered epithelium.

✵ Absence of a skull and any vertebral rudiments.

✵ Absence of paired limbs.

✵ The Muscular System allows only simple, repetitive movements: bending the body from side to side.

✵ The digestive tract is poorly differentiated.

✵ The respiratory organs consist of gill slits that perforate the pharynx.

✵ Primitive structure of the excretory organs.

✵ Absence of a brain and a Heart.

✵ Primitive sensory organs: light-sensitive cells, an olfactory pit, and tactile receptors.

✵ Numerous gonads that lack their own ducts.

The Role of cephalochordates in nature:

1. In aquatic ecosystems, cephalochordates serve as a link in the food chain.

2. In the Evolution of the animal kingdom, cephalochordates represent an intermediate stage between lower and higher animals.

3. They act as filter feeders in ecosystems, regulating the Abundance of planktonic organisms.

Significance of cephalochordates to humans:

1. The lancelet is a convenient subject for scientific research.

2. In Southeast Asia, lancelets are used as a food source.