第十五章 低等的无颌脊椎动物 — 圆口纲（ Cyclostomata ）

1. 第十五章 低等的无颌脊椎动物—圆口纲（Cyclostomata）

2. CYCLOSTOMATA • cyclostomata is vertebrates without jaws. • The living members of the cyclostomata are represented(记载) by some 50 species almost equally divided between two classes: Petromyzontes(七鳃鳗）) and Myxini（盲鳗） . They have in common the absence of jaws, internal ossification(骨骼). scales, and paired fins and both share pore-like gill openings and an eel-like body form. At the same time there are so many important differences, some of which are indicated in the listing below, that they have been assigned to separate vertebrate classes.

3. 圆口纲动物又称无颌类，因有一个圆形的口吸盘而得名为圆口类，是无颌、无成对附肢的低等脊椎动物，主要包括七鳃鳗和盲鳗两大类。生活在海水或淡水中，外形象鱼，但比鱼类低等。由于营寄生或半寄生，成为危害渔业的动物。全世界约50种。圆口纲动物又称无颌类，因有一个圆形的口吸盘而得名为圆口类，是无颌、无成对附肢的低等脊椎动物，主要包括七鳃鳗和盲鳗两大类。生活在海水或淡水中，外形象鱼，但比鱼类低等。由于营寄生或半寄生，成为危害渔业的动物。全世界约50种。

5. Characteristics • 1. Body slender, eel-like, rounded, with soft skin containing mucous(黏液) glands but no scales • 2. Median fins with cartilaginous(软骨) fin rays, but no paired appendages(附肢) • 3. Fibrous and cartilaginous skeleton; notochord persistent • 4. Sucker-like oral disk with well-developed teeth in lampreys; mouth with two rows of eversible(可外翻的) teeth in hagfish • 5. Heart with one auricle (心房)and one ventricle(心室); aortic arches in gill region; blood with erythrocytes(红血球) and leukocytes (白血球) • 6. Seven pairs of gills in lampreys; 5 to 15 pairs of gills in hagfish

6. 7. Mesonephric kidney in lampreys（七鳃鳗）; pronephric kidney（原肾） anteriorly and mesonephric kidney（中肾） posteriorly in hagfish（盲鳗） • 8. Dorsal nerve cord with differentiated brain; 8 to l0 pairs of cranial nerves • 9. Digestive system without stomach; intestine(肠) with spiral fold in lampreys; spiral fold absent in hagfish • l0. Sense organs of taste(味觉), smell, hearing, eyes moderately developed in lampreys but highly degenerate（退化） in hagfish • 11. External fertilization: gonad single without duct; sexes separate and long larval stage in lampreys; hermaphroditic (雌雄同体)and direct development with no larval stage in hagfish Class Petromyzontes—lampreys

7. 主要特征 • 1 无上下颌； • 2 无成对附肢(偶鳍)； • 3 鼻孔单个，位于头部背面。 • 4 鳃位于咽部两侧的鳃囊中。 • 5 生殖腺单个，无输出管。

8. 原始和特化的结构 • 无成对的附肢即偶鳍，仅有奇鳍，尾是脊椎动物中最原始的原尾型。 • 无主动捕食的可咬合的上下颌。具适应寄生半寄生生活的可吸附的口漏斗和角质齿的舌。 • 具原始不分节的肌节，成W形，角顶朝前。 • 具有特殊的鳃囊和内鳃孔，由特化的软骨鳃篮支持。 • 具有一对唾腺

9. 进步且不完善的结构 • 皮肤由多层上皮细胞组成，有发达的单胞腺，真皮为结缔组织。 • 支持结构为脊索，但已有雏形的脊椎骨。（脊索背面的每1个体节出现2对极小的软骨弧片，代表雏形脊椎骨。） • 背神经管分化为脑和脊髓，脑分化为大脑、间脑、中脑、小脑和延脑5部分，但依次排列在一个平面上。

10. 头骨原始，仅为脑下方的软骨基板。 • 有集中的感觉器官：嗅觉器官、听觉器官、视觉器官。 • 心脏分化为1心房、1心室、1静脉窦组成，血液中已有红细胞。单循环。 • 具集中的肾脏和生殖腺。成体肾脏为背肾，胚胎期为前肾，但盲鳗成体仍保留前肾。雌雄异体，无生殖管道。

11. All the lampreys of the northern hemisphere belong to the family Petromyzontidae. The destructive marine lamprey Petromyzon marinus is found on both sides of the Atlantic Ocean (America and Europe) and may attain a length of 3 feet. Lampetra also has a wide distribution in North America and Eurasia and ranges from 6 to 24 inches long. There are l9 species of lampreys in North America. About half of these belong to the nonparasitic brook type; the others are parasitic. The nonparasitic species have probably arisen from the parasitic forms by degeneration of the teeth, alimentary canal, etc. They may have done so by evolving from the parasitic species through pedomorphosis (reproduction by young, especially by parthenogenesis). The genus Ichthyomyzon. which contains three parasitic and three nonparasitic species, is restricted to eastern North America. On the west coast of North America the chief marine form is represented by Lampetra (Entospbenus) tridentatus.

12. All lampreys, marine as well as freshwater forms, spawn(产卵) in the spring in North America (autumn in some European species) in shallow gravel and sand in freshwater streams. The males begin nest building and are joined later by females. At spawning（产卵）, with the female attached to a rock to maintain position over the nest, the male attaches to the dorsal side of her head. He curls(卷曲) the posterior part of his body tightly about that of the female . As the eggs are shed into the nest, they are fertilized by the male. Only small numbers of eggs are shed at each mating(交配), and the process may be repeated numerous times over a period of l to 2 days. The sticky(粘性) eggs adhere(附着) to pebbles(小圆石) in the nest and soon become covered with sand. The adults die soon after spawning.

13. Parasitic lampreys either migrate to the sea, if marine, or else remain in freshwater, where they attach themselves by their sucker-like mouth to fish and, with their sharp horny teeth, rasp away the flesh and suck out the blood. To promote the flow of blood, the lamprey injects an anticoagulant(抗血凝素) into the wound. The parasitic freshwater adults live a year or more before spawning and then die; the marine forms may live longer. • The nonparasitic lampreys do not feed after emerging as adults, for their alimentary canal (消化道) degenerates(退化) to a nonfunctional strand of tissue. Within a few months they also spawn and die.

14. 结构和功能(七鳃鳗的成体) • 外形： • 盲鳗（A）和七鳃鳗（B） • 体长15cm至1 m，分头、躯干和尾。皮肤裸露无鳞，皮肤中单细胞粘液腺发达。无偶鳍，只有奇鳍，即背鳍和尾鳍(雌性具一臀鳍)。1对眼，无眼睑。眼后两侧各有7个鳃裂开孔，故名七鳃鳗，过去有人误认为鳃裂也是眼睛，又有八目鳗之称。 • 身体前端具口漏斗，尾前部腹面有肛门和泄殖孔。营寄生或半寄生，以口漏斗吸附于寄主体表或钻人体内。

16. 内部结构 • 皮肤： • 皮肤裸露，皮肤中的表皮已由多层上皮细胞组成，内有发达的单细胞腺，分泌粘液使体表粘滑。真皮为有规则排列的结缔组织，有韧性。 • 骨骼系统： • 身体主要支持结构仍为脊索，但已出现了雏形的脊椎骨脊索背面的每1个体节出现2对极小的软骨弧片，无任何支持作用，但代表雏形脊椎骨。

17. 呼吸系统： • 咽分背、腹两管，背部为消化管，腹部为呼吸管。 • 通路：两条，外鳃孔周围具有肌肉，可控制水流的出入。 • 水流→口腔→呼吸道→内鳃孔→鳃囊 • ↓↑ • 外鳃孔（7个） • ↓↑ • 水流 • 鳃囊（gill pouch）是鳃裂的一部分膨大形成的，囊壁上由若干褶皱状鳃丝(gill filament)。此外，鳃囊两端有两小管，近呼吸道的叫内鳃管（入），近外鳃孔的叫外鳃管（出），两管内不具鳃丝，故圆口类又称囊鳃类。

19. 肌肉： • 具有原始肌节，成W形，角顶朝前。 • 消化系统： • 无主动捕食的可咬合的上下颌。由于适应寄生、半寄生生活而具有可吸附的口漏斗和具有角质齿的舌，以此吸附在鱼体上并刺破鱼的皮肤而吸食血肉。口腔腺分泌抗凝血剂，使寄主血液不凝固。无消化道各部的分化。

20. 循环系统： • 已有心脏的分化，由1心房、1心室、1静脉窦组成，血液中已有红细胞。单循环，循环路线与文昌鱼相似，有肝门静脉。 • 神经系统： • 神经：背神经管分化为脑和脊髓，脑又进一步分化为大脑、间脑、中脑、小脑和延脑5部分，但依次排列在一个平面上。 • 感官：有集中的感觉器官。嗅觉器官为单个的外鼻孔；听觉器官仅有内耳的前、后半规管盲鳗仅具1个半规管)；视觉器官为1对眼，眼已具有脊椎动物眼的基本模式，但由于生舌方式所限，眼不发达。具有原始的松果眼和顶眼，仅有感光作用。

22. 排泄系统： • 具有集中的肾脏和生殖腺。肾脏1对，呈长条形。成体中肾，胚胎前肾。 • 生殖系统： • 雌雄异体，生殖腺单个，无生殖管道，成熟的精子和卵子进入体腔，经泄殖孔排出体外。

23. Myxini--hagfishes • The hagfishes are an entirely marine group that feed on dead or dying fishes, annelids, mollusks, and crustaceans. Thus they are neither parasitic like lampreys nor predaceous(肉食性), but are scavengers（清道夫）. There are only 2l described species of hagfishes, of which the best known in North America are the Atlantic hagfish Myxine glutinosa（盲鳗） and the Pacific hagfish Bdellostoma stouti（粘盲鳗）.

24. Hagfishes have long been of interest to comparative physiologists. Unlike any other vertebrate, they are isosmotic（等渗透压） to sea water like marine invertebrates. They are the only vertebrates to have both pronephric and mesonephric kidneys in the adult, although only the latter forms urine. They have no less than four sets of hearts positioned at different places in the body to boost blood flow through their low-pressure circulatory system. Despite other unique anatomic and physiologic features of interest to biologists.

25. The yolk-filled egg of the hagfish may be nearly l inch in diameter and is enclosed in a horny (角状的)shell. There is no larval stage and growth is direct. They are hermaphroditic but can produce only one kind of gamete（配子） at a time; a single individual may produce sperm at one season and eggs the next.

27. 七鳃鳗与文昌鱼的比较

28. 总 结 圆口纲动物是最原始的无颌脊椎动物，包括七鳃鳗和盲鳗。身体鳗形，无成对附肢，具软骨（虽然其祖先甲胄鱼具有硬骨骼），脊索终生存在，有雏形脊椎骨。具口吸盘以吮吸方式取食，营寄生或半寄生生活。寒武纪晚期底栖的甲胄鱼类与圆口类有共同的祖先。

29. 思 考 题 1.以七鳃鳗的结构特点说明它是最原始的脊椎动物。 2.甲胄鱼的进化地位及与圆口类的关系。