Arrow-shaped

	Arrow-shaped
	; Sagitta Front End
Scientific classification
Domain:	Eukaryotes
Kingdom:	Animals
Kingdom :	Eumetazoi
No rank :	Bilateral symmetrical
Type of:	Maxillary
Grade:	Sagittoidea
International scientific name
	Sagittoidea Claus & Grobben, 1905
Detachments
	Aphragmophora; Phragmophora;

Arrows ( lat. Sagittoidea ) - a class of marine invertebrates of the type of bristle-maxillary.

Content

Appearance and structure

The arrow-shaped ones have a transparent body of elongated arrow-shaped form (hence their name). The length of the adult organism is from 5 to 90 mm. These animals lack the respiratory, circulatory, excretory systems, but at the same time they have signs of high organization: multilayered epithelium and striated muscles.

The bodies of the arrow-shaped are divided into the head, trunk and tail. They are separated from each other by two transverse partitions. There are 1-2 pairs of fins on the sides of the body, and one caudal at the posterior end. These are skin folds with rays. The digestive system consists of a mouth located on the underside of the head, pharynx, intestine, and anus opening between the trunk and tail.

To grab food, they have a hunting apparatus of long hook-shaped bristles, on the sides of the head, and 1-2 rows of small cloves in front of the head at the mouth. There are two nerve nodes: in the head and trunk. Two eyes are located on the head.

Lifestyle & Nutrition

Planktonic organisms. Predators They eat crustaceans, fish fry, their own young, as well as algae. Compete for prey with young fish.

Reproduction and development

Sharp-shaped bisexual floors. The ovaries are located in the back of the trunk, and the testes in the tail. Sperm go out through special seminal vesicles located on the sides of the caudal region between the posterior lateral and caudal fins. Cross fertilization. There is no larval stage in development.

Systematics

- Squad Aphragmophora
  - Family Bathybelidae
    - Genus Bathybelos
  - Krohnittidae Family
    - Genus Krohnitta
  - Pterokrohniidae Family
    - Genus Pterokrohnia
  - Pterosagittidae Family
    - Genus Pterosagitta
  - Sagittidae Family
    - Genus Aidanosagitta
    - Genus Caecosagitta
    - Genus Decipisagitta
    - Genus Ferosagitta
    - Genus Flaccisagitta
    - Genus Mesosagitta
    - Genus Parasagitta
    - Genus Pseudosagitta
    - Genus Sagitta
    - Genus Serratosagitta
    - Genus Solidosagitta
    - Genus Zonosagitta
- Squad Phragmophora
  - Eukrohniidae Family
    - Genus Eukrohnia
  - Heterokrohniidae Family
    - Genus Archeterokrohnia
    - Genus Heterokrohnia
    - Genus Xenokrohnia
  - Krohnittellidae Family
    - Genus Krohnittella
  - Spadellidae Family
    - Genus Bathyspadella
    - Genus Calispadella
    - Genus Hemispadella
    - Genus Paraspadella
    - Genus Spadella

Literature

Animals and plants of Peter the Great Bay / Zhirmunsky A.V. - Leningrad: Science, 1976. - 364 p.