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CHAETOGNATHA

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Originally appearing in Volume V05, Page 789 of the 1911 Encyclopedia Britannica.

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CHAETOGNATHA , the name given by R. Leuckhart to a small See also:

GROUP

group of transparent and for the most See also:

PART

part pelagic organisms, whose position in the See also:

animal See also:

KINGDOM

kingdom is a very isolated one. Only three genera; See also:

SAGITTA (" the arrow " or " dart ")

Sagitta, Spadella and Krohnia, are recognised, and the number of See also:

SPECIES

species is small. Nevertheless these animals exist in extraordinary quantities, so that at certain seasons and under certain conditions the See also:

SURFACE

surface of the See also:

sea seems almost stiff with the incredible multitude of organisms which pervade it. Rough seas, &c., cause them to seek safety in dropping into deeper See also:

WATER

water. Deep-sea forms also occur, but in spite of this the group is essentially pelagic. As a See also:

RULE

rule the See also:

BODY

body is some r to 2 or 3 cm. in length, though some species are larger, by 4 or 5 mm. in breadth, and it is shaped rt something like a See also:

TORPEDO

torpedo with See also:

side flanges -__he and a slightly swollen, rounded See also:

head. It s can be divided into three regions—(i.) head, f (ii.) See also:

TRUNK (Fr. tronc, Lat. truncus, cut off, maimed)

trunk, and (iii.) tail, separated from one another by two transverse septa. The almost spherical head is covered by a See also:

hood which can be retracted; it bears upon its J side a number of sickle-shaped, chitinous hooks and one or more See also:

SHORT, FRANCIS JOB (1857– )

short rows of See also:

low °9 spines—both of these features are used in characterizing the various species. A pair of eyes See also:

lie dorsally and behind them Is a closed circlet, often pulled out into various shapes, of modified epidermis, to which an olfactory See also:

FUNCTION

function has been attributed. The interior of the head is filled up with masses of muscle See also:

fibres which are mainly occupied with moving the sickle-shaped hooks. The r s trunk contains a spacious body-cavity filled during the breeding See also:

SEASON (0. Fr. seson, seison, mod. saison, Lat. satio, sowing time, the spring, from serere, to sow; in Late Lat. the word is found with its present meaning, the spring being considered as particularly the season of the year)

season by the swollen ovaries, and the same is true of the tail if we substitute testes for ovaries.

The skin consists of a transparent cuticle excreted by the underlying ectoderm, the Spadella cephaloptera cells of which though usually one-layered may be heaped up into several layers in (See also:

Busch). the head; beneath this is a See also:

BASEMENT

basement St, Septa dividing membrane, and then a layer of See also:

LONGITUDINAL

longitudinal body-cavity trans- muscle fibres which are limited inside by a versely. layer of peritoneal cells. The muscles are Cerebral ganglia. striated and arranged in four quadrants, Commissure unit- two dorso-lateral and two ventro-lateral, See also:

ing this with ven- an arrangement which recalls that of the tral ganglion (not See also:

NEMATODA

Nematoda, whilst in their See also:

histology they shown in fig.). somewhat resemble the muscles of the n2, See also:

NERVE (Lat. nervus, Gr. vevpov, a bowstring)

Nerve uniting sere- See also:

OLIGOCHAETA

Oligochaeta. Along each side of the body bral ganglia with stretches a See also:

HORIZONTAL

horizontal fin and a similar small ganglia on flange surrounds the tail. Into these fins, head. which are largely cuticular and strengthened nr, Olfactory nerve. by radiating bars, a single layer of ectoderm d, Alimentary See also:

canal. cells projects. r, Olfactory See also:

ORGAN

organ. The mouth, a longitudinal slit, opens on te, Tentacle. to the ventral surface of the head. It leads t, Tactile hairs See also:

SPRING (from " to spring," " to leap or jump up," " burst out," O. Eng., springau, a common Teut. word, cf. Ger. springer, possibly allied to Gr. QnrFpxecOac, to move rapidly)

spring- into a straight alimentary canal whose walls ing from surface consist of a layer of ciliated cells ensheathed of body. in a thin layer of peritoneal cells. There is e, Ovary. no See also:

ARMATURE (from Lat. armatura, armour)

armature, and no glands, and the whole el, Oviduct. See also:

TRACT (from Lat. tractare, to treat of a matter, through Provencal tractat and Ital. trattato)

tract can only be divided into an See also:

OESOPHAGUS (Gr. o'lvca=I will carry, and r/)ayeiv, to eat)

oesophagus ho, Testes. and an See also:

INTESTINE (Lat. intestinus, internal, usually in neuter plural intestina. from intus, within)

intestine. The latter runs with no sg, Vas deferens. [fins. twists or coils straight to the anus, which is See also:

F2(X2+Y2)

f2,f',Lateral and caudal situated at the junction of the trunk with sb, Seminal pouch. the tail. A median mesentery See also:

RUNNING

running dorso-The eyes are indi- ventrally supports the alimentary canal and cated as See also:

black dots is continued behind it into the tail, thus behind the cerebral dividing the body cavity into two lateral ganglia. halves. There are no specialized circulatory, See also:

RESPIRATORY

respiratory or excretory See also:

ORGANS

organs.

The See also:

NERVOUS

nervous See also:

SYSTEM

system consists of a cerebral ganglion in the head,a conspicuous ventral ganglion in the trunk, and of lateral cornmissures uniting these ganglia on each side. The whole of this system has retained its See also:

PRIMITIVE

primitive connexion with the ectoderm. The cerebral ganglion also gives off a nerve on each side to a pair of small- ganglia, See also:

UNITED

united by a median commissure, which have sunk into and See also:

CONTROL (Fr. contrdle, older form contre rolle, from Med. Lat. contra-rotulus, a counter roll or copy of a document used to check the original; there is no instance in English of the use of " control " in this, its literal, meaning)

control the muscles of the head. As in other animals there is a See also:

minute but extensive nervous plexus, which permeates the whole body and takes its origin from the See also:

CHIEF (from Fr. chef, head, Lat. caput)

chief ganglia. In addition to the eyes and the olfactory circle on the head scattered tactile papillae are found on the ectoderm. Chaetognatha are hermaphrodite. The ovaries are attached to the side walls of the trunk region; between them and the body See also:

wall lie the two oviducts whose inner and anterior end is described as closed, their See also:

OUTER

outer ends opening one on each side of the anus, where the trunk joins the tail. According to See also:

MISS

Miss N. M. See also:

Stevens the so-called oviduct acts only as a " sperm-duct " or receptaculum seminis. The spermatozoa enter it and pass through its walls and See also:

TRAVERSE

traverse a minute duct formed of two See also:

ACCESSORY

accessory cells, and finally enter the ripe ovum. Temporary oviducts are formed between the " sperm-See also:

DUET (an adaptation of the Ital. duetto, from Lat. duo, two)

duet " and the germinal epithelium at each oviposition.

A number of ova ripen simultaneously. The two testes lie in the tail and are formed by lateral proliferations of the living peritoneal cells. These break off and, lying in the coelomic fluid, break up into spermatozoa. They pass out through short See also:

VASA, or NIKOLAISTAD

vasa deferentia with See also:

INTERNAL

internal ciliated funnels, sometimes an enlargement on their course—the seminal vesicles—and a minute See also:

EXTERNAL

external See also:

PORE

pore situated on the side of the tail. With hardly an exception the transparent eggs are laid into the sea and See also:

FLOAT (in O. Eng. floc and flota, in the verbal form f eotan; the Teutonic root is flut-, another form of flu-, seen in " flow," cf. " fleet "; the root is seen in Gr. a-M e, to sail, Lat. pluere, to rain; the Lat, fluere and fluctus, wave, is not connect

float on its surface. The development is See also:

DIRECT

direct and there is no larval See also:

STAGE (Fr. 6/age; from Lat. stare, to stand)

stage. The segmentation is See also:

COMPLETE

complete; one side of the hollow blastosphere invaginates and forms a gastrula. The blastopore closes, a new mouth and a new anus subsequently arising. The archenteron gives off two lateral pounchs and thus becomes trilobed. The See also:

MIDDLE

middle See also:

LOBE

lobe forms the alimentary canal; it closes behind and opens to the exterior anteriorly and so makes the mouth. The two lateral lobes contain the coelom; each separates off in front a segment which forms the head and presumably then divides again to See also:

FORM (Lat. forma)

form anteriorly the trunk, and posteriorly the tail regions. An interesting feature of the development of Chaetognaths is that, as in some See also:

INSECTS

insects, the cells destined to form the reproductive organs are differentiated at a very See also:

early See also:

period, being apparent even in the gastrula stage.

The See also:

GREAT

great bulk of the group is pelagic, as the transparent nature of all their tissues indicates. They move by flexing their bodies. Spadella cephaloptera is, however, littoral and oviposits on sea-See also:

WEED, THURLOW (1797—1882)

weed, and the " See also:

VALDIVIA

Valdivia " brought See also:

home a deep-sea species. The three genera are differentiated as follows: Sagitta M. Slabber, with two pairs of lateral fins. This genus was named as See also:

long ago as 1775. Krohnia P. Langerhans, with one lateral fin on each side, extending on to the tail. Spadella P. Langerhans, with a pair of lateral fins on the tail and a thickened ectodermic See also:

ridge running back on each side from the head to the anterior end of the fin. The group is an isolated one and should probably be regarded as a See also:

SEPARATE

separate phylum. It has certain histological resemblances with the Nematoda and certain primitive Annelids, but little stress must be laid on these.

The most that can be said is that the Chaetognaths begin See also:

LIFE

life with three segments, a feature they See also:

SHARE (O. Eng. scearu, chiefly in compounds, e.g. land-scearu, a share of land, from sceran to cut; cf. " shear" )

share with such widely-differing See also:

groups as the See also:

BRACHIOPODA

Brachiopoda, the See also:

ECHINODERMA

Echinoderma and the Enteropneusta, and probably See also:

VERTEBRATA

Vertebrata generally. See O. Hertwig, See also:

Die Chaetognathen, eine Monographie (See also:

JENA

Jena, 188o) ; B. J. Grassi, Chetognathi: See also:

FLORA

Flora u. See also:

FAUNA

Fauna d. Golfes von Neapel (1883) ; S. Strodtman, See also:

Arch. Naturg. lviii., 1892; N. M. Stevens, Zool. Jahrb.

Anat. xviii., 1903, and xxi., 1905. (A. E.

End of Article: CHAETOGNATHA

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