The zooids are a modification of the type of structure known in Balanhglossus, from which they differ principally in the following respects: (i.) The alimentary canal, instead of being straight, has a U-shaped flexure, the dorsal line between the mouth and the anus being short.
The remarkable position of the anus (a) on the dorsal side has already been alluded to.
The mouth opens at one extremity of the body and the anus at or near the other.
The alimentary tract consists of a straight tube running from the mouth to the anus without any convolutions; it is separable into three divisions: (I) a muscular oesophagus, which is often provided with cuticular teeth; (2) a cellular intestine; and (3) a short terminal rectum surrounded by muscular fibres.
Many beetles have, in connexion with the anus, glands which secrete a repellent acid fluid, serving as a defence for the insect when attacked.
Commonly the nephridia are strictly paired a single pair to each segment, while the branches of the blood vascular system are similarly metameric. The alimentary canal is nearly always a straight tube running from the mouth, which is surrounded by the first segment of the body and overhung by the prostomium, to the anus,which is then either surrounded by the last segment of the body or opens dorsally a little way in front of this.
The anus is mostly terminal, and there are no anterior and posterior suckers.
Cobangia, the anus is anterior and ventral.
Ment bearing male pores; 32, 37, Anus nearly always ter-, Acanthod firstand last segments of clitellum.
- The alimentary canal is always a straight tube, and the anus, save in the genera Criodrilus and Dero, is completely terminal.
The anus is dorsal.
At the posterior end of the body there are likewise seven separate ganglia partially fused to form a single ganglionic mass, which innervates the segments lying behind the anus and corresponding to the posterior sucker.
In development, the openings of the mantle-cavity and the anus are always originally posterior; later they are brought forward ventrally.
The second movement is a lateral torsion of the visceral mass, the foot remaining a fixed point; this torsion occurs in a plane approximately at right angles to that of the first movement, and carries the pallial aperture and the anus from behind forwards.
Finally, the original symmetry of the circumanal complex vanishes; the anus leaves the centre of the pallial cavity and passes towards the right side (left side in sinistral forms); the organs of this side become atrophied and disappear.
It is followed in some specialized Heteropoda and in the Euthyneura by a torsion in the opposite direction, or detorsion, which brings the anus farther back and untwists the visceral commissure (see Euthyneura, below).
Here it lies close upon the genital body (ovary or testis), and in such intimate relationship with it that, when ripe, the gonad bursts into the renal sac, and its products are carried to the exterior by the papilla on the right side of the anus is FIG.
It should be to the right of the anus were this the case.
There is but very little food-material in the egg of this Pectinibranch, and consequently the diblastula forms by invagination; the blastopore or orifice of invagination coincides with the anus, and never closes entirely.
The general features of this process and of the relation of the blastopore to mouth and anus have been explained in treating of the development of Mollusca generally.
In such cases the blastopore may entirely close, and both mouth and anus develop as new ingrowths (stomodaeum and proctodaeum), whilst, according to the observations of N.
Stylifer, the operculum is lost, animal fixed by a large proboscis which forms a pseudopallium covering the whole shell except the extremity of the spire, parasitic on all groups of Echinoderms. Entosiphon, visceral mass still coiled; shell much reduced, proboscis very long forming a pseudopallium which covers the whole body and projects beyond in the form of a siphon, foot and nervous system present, eyes, branchia and anus absent, parasite in the Holothurian Deima blakei in the Indian Ocean.
No shell; visceral mass not coiled; no sensory organs, nervous system, branchia or anus; body reduced to a more or less tubular sac; hermaphrodite and viviparous; parasitic in Holothurians; larvae are veligers, with shell and operculum.
Visceral sac very much reduced; without shell or mantle; anus posterior; foot provided with sucker in male only.
Sub-Class II.-Euthyneura The most important general character of the Euthyneura is the absence of torsion in the visceral commissure, and the more posterior position of the anus and pallial organs.
But in the other members of the sub-class the detorsion of the visceral mass has carried back the anus and circumanal complex from the anterior dorsal region to the right side, as in Bulla and Aplysia, or even to the posterior end of the body, as in Philine, Oncidium, Doris, &c. Different degrees of the same process of detorsion are, as we have seen, exhibited by the Heteropoda among the Streptoneura, and both in them and in the Euthyneura the detorsion is associated with degeneration of the shell.
The anus is placed in such forms far back beyond the mantle-skirt.
In front of the anus, and only partially covered ry FIG.
The torsion of the visceral hump is not carried out very fully, the consequence being that the anus has a posterior position a little to the right of the median line above the metapodium, whilst the branchial chamber formed by the overhanging mantle-skirt faces the right side of the body instead of lying well to the front as in Streptoneura and as in Pulmonate Euthyneura.
Posteriorly we have the anus, in front of this the lobate gill-plume, between the two (hence corresponding in position to that of the Pectinibranchia) we have the aperture of the renal organ.
Anus lateral, on the right side.
Body externally symmetrical; anus median, posterior, and generally dorsal, surrounded by ramified pallial appendages, constituting a secondary branchia.
Anus and branchia posterior, below the mantle-border.
The anus is antero-lateral, except in the Proctonotidae, in which it is median.
Head without tentacles; body elongated; anus on right side.
Anus posterior, median; anterior tentacles, atrophied; foot broad.
No lateral expansions, and no dorsal papillae; body planariform; anus dorsal, median and posterior.
60), at one time supposed to be the developing anus, but shown by Lankester to be identical with the " shell-gland " discovered by him in other Mollusca (Pisidium, Pleurobranchidium, Neritina, &c.).
No shell; limaciform; littoral; female aperture posterior, near anus; a reduced pulmonary cavity with a distinct aperture.
A, anus; Abx 1 Abx 11, appendage of 1st and of 11th abdominal segments; Ans, anal piece=telson or 12th abdominal segment; Ant, antenna; De, deuterencephalon; Md, mandible; Mx1, first maxilla; Mx2, second maxilla or labium; 0, mouth; Obcl, rudimentary labrum and clypeus; Pre, protencephalon; St t, Stu), stigmata i and io; Terg, tergite; Thx1, appendage of first thoracic segment; Tre, tritencephalon; Ul, a thickening at hinder margin of the mouth.
The embryonic ectoderm of an insect consists of a layer of cells forming a continuous structure, the orifices in it - mouth, spiracles, anus and terminal portions of the genital ducts - being invaginations of the outer wall.
In nearly all Nemertines the rhynchocoel extends backwards as far as the posterior extremity, just above the anus; in Cannella it is limited to the anterior bodyregion.
In certain Metanemertines the lateral stems have been noticed to unite posteriorly by a terminal commissure, situated above the anus, the whole of the central nervous system being in this way virtually situated above the intestine.
The anus is situated ter m, P9 minally, the muscular body wall through which the ?
The lateral nerve stems mostly terminate on both sides in closest prox N imity to the anus; in cer tain species, however, they interfuse by a transverse connexion above the anus.
The alimentary canal is a simple tube traversing the body from end to end, the anus opening at the extremity of its narrowed tail-like termination.
These and the oviducts lie on the anterior half of the body; but the oviducts themselves soon unite to form a single tube of great length, which runs backwards to its posterior extremity, terminating in the genital orifice close to the anus, In the male, on the contrary, this orifice is situated in the anterior half of the body, not far behind the mouth.
At the same time some Acari, like Eriophyes (Phytoptus) and Demodex, have the body elongated and annulated, but in these groups the elongation of the body is caudal or post-anal, as is attested by the position of the anus far forwards on its ventral surface.
The mouth opens into an oesophagus which passes into an intestine; this opens by a ventral anus situated a little in front of the posterior end.
The last is perforated by the anus and carries the post-anal spine or sting.