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 metasomatic cavities are divided by dorsal (fig.
The most conspicuous blood vessel possessed by Cephalodiscus is the dorsal vessel (d.b.v.).
At their opposite ends the dorsal and ventral vessels are probably connected with one another by means of a splanchnic sinus surrounding the stomach.
D.b.v., Dorsal blood-vessel.
The eggs of Cephalodiscus d.mes.,Dorsal mesentery.
Turning to the thorax we find that the first segment (prothorax) is distinct and free, with a wide dorsal sclerite.
Tail long, its apical half-clothed on the dorsal surface with long hairs.
HAKE (Merluccius vulgaris), a fish which differs from the cod in having only two dorsal fins and one anal.
Beneath the epidermis is a longitudinal layer of muscle-fibres which are separated into four distinct groups by the dorsal, ventral and lateral areas; these are occupied by a continuation of the epidermic layer; in the lateral areas run two thin-walled tubes with clear contents, which unite in the anterior part of the body and open by a pore situated on the ventral surface usually about a quarter or a third of the body length from the anterior end.
These processes stretch across the body cavity to be inserted in the dorsal and ventral middle lines.
S, Opening of segmental tubes (placed by mistake on the dorsal instead of the ventral surface).
In other species, however, a peculiar type of polystely is met with, in which the original diarch stele gives rise to se-called dorsal and ventral stelar cords which at first lie on the surface of the primary stele, but eventually at a higher level separate from it and form distinct secondary steles resembling the primary one.
This is due to the fact that while young the turgidity and consequent growth are greater in the dorsal side of the leaf, so that it becomes rolled up. As it develops the maximum turgidity and growth change to its upper side, and so it becomes unfolded or expanded.
The ethmoid frequently appears on the dorsal surface between the frontals.
The dorsal or hyomandibular portion of this same arch is transformed into the auditory chain, ending in the fenestra ovalis.
The atlas is composed of three pieces; a pair of lateral ele ptz pt.z W " ments (the right and left dorsal arch pieces) joining above the spinal cord, and a ventral piece equivalent to the first basiventral elements, i.e.
Dorsal vertebrae frequently have a ventral outgrowth of the centrum; these hypapophyses may be simple vertical blades, I-shaped, or paired knobs; they serve for the attachment of the thoracic origin of the longus collianticus muscle, reaching their greatest development in Sphenisci and Colymbidae.
There is a well-marked processus lateralis anterior (the right and left together equivalent to the mammalian manubrium), which is the product of two or three ribs, the dorsal parts of which reduced ribs remain as cervico-dorsal ribs.
The ulna is curved and rather stout; it articulates with both carpal bones; the cubital quills often cause rugosities on its dorsal surface.
The ventral inner margin of the preacetabular portion of the ilium is attached to the pre-sacral vertebrae, whilst the inner and dorsal margin of the postacetabular portion is attached to the primary sacral and the postsacral vertebrae.
Their transverse dorsal connexion is the posterior commissure; otherwise the whole roof portion of the midbrain is reduced to a thin membrane, continuous with that which covers the Sylvian aqueduct, and this ventricle sends a lateral cavity into each optic lobe, as is the case in reptiles.
This is chiefly cartilaginous and sends out three processes: the dorsal (s.
The lungs are small and occupy only the dorsal portion of the thoracic cavity.
Further, according to these muscles being inserted only upon the dorsal, or only upon the ventral, or on both ends of the semi-rings, we distinguish between an-, kat- and diacromyodi.
On the other hand, the diacromyodian type can have been developed only from a strong muscular basis which could split into a dorsal and a ventral mass; moreover, no Passeres are known to be intermediate between those that are diacromyodian and those that are not.
There remains but one logical way, namely, to distinguish as follows: - (i) Passeres anisomyodi, in which the syrinx muscles are unequally inserted, either on the middle or on one end of the semi-rings, either dorsal or ventral.
(2) Passeres diacromyodi, in which some of the syrinx muscles are attached to the dorsal, and some to the ventral ends, those ends being, so to say, equally treated.
It lodges the copulatory organ, and on its dorsal wall lies the bursa Fabricii, an organ peculiar to birds.
It occurs in two different forms. In the Ratitae, except Rhea, it consists mainly of a right and left united half (corpora fibrosa), with a deep longitudinal furrow on the dorsal side, and much resembles the same organ in crocodiles and tortoises.
- Syrinx muscles entirely lateral or attached to the dorsal or ventral corners of the bronchial semi-rings.
- Syrinx muscles of either side attached to the dorsal and ventral corners of the rings.
The ventral and anal fins are white, tinged with pale red; and the dorsal, pectoral and caudal tipped with black.
1, A), are usually convex above, with straight hind margins (dorsa); when the elytra are closed, the two hind margins come together along the mid-dorsal line of the body, forming a suture.
A, Dorsal view; B, mouth organs; C, under side.
Among the longhorn beetles, the prothorax scrapes over a median file on the mid-dorsal aspect of the mesothorax.
The fifth abdominal segment has a pair of strong dorsal hook-like processes, by means of which the larva supports itself in the burrow which it excavates in the earth, the great head blocking the entrance with the mandibles ready to seize on any unwary insect that may venture within reach.
Io) the whole dorsal region of the abdomen, but sometimes leaving as many as four terga exposed (fig.
When Hydrophilus dives it carries a supply of air between the elytra and the dorsal surface of the abdomen, while air is FIG.
After eating the contents of the egg, the larva moults and becomes a fleshy grub with short legs and with paired spiracles close to the dorsal region, so that, as it floats in and devours the honey, it obtains a supply of air.
Its body is well-proportioned, rather elongate, and somewhat like that of the European barbel, but covered with very large scales, of which there are only twentyfive or twenty-seven placed along the lateral line; the dorsal fin is armed with a long and strong spine, and the mouth provided with four slender and short barbels.
All the fins have a rounded outline; the short dorsal fin is without a spine, but the males possess a very thick and flattened outer ray in the ventral fins.
On the other hand, in certain Polychaeta the bundles of setae are so extensive that they nearly form a complete circle surrounding the body; and in the Oligochaet genus Perichaeta (=Pheretima), and some allies, there is actually a complete circle of setae in each segment broken only by minute gaps, one dorsal, the other ventral.
Among the Oligochaeta the dorsal vessel in Dinodrilus and Megascolides is enclosed in a separate coelomic chamber which may or may not communicate with the main coelomic cavity.
In the smaller and simpler forms the capillary networks are much reduced, but the dorsal and ventral vessels are usually present.
On the other hand, additional longitudinal trunks are sometimes developed, the chief one of which is a supra-intestinal vessel lying below the dorsal vessel and closely adherent to the walls of the oesophagus in which region it appears.
Among many Oligochaeta the dorsal blood-vessel is partly or entirely a double tube, which is a retention of a character shown by F.
In terrestrial A, Lumbricus : 9, to, segments contain ing spermathecae, the orifices of forms dorsal pores are usuwhich are indicated; 14, segment b ally present; in aquatic earing oviducal pores; 15, segforms a head pore only.
It is possible, however, that those oviducts belong to a separate morphological category, more comparable to the dorsal pores and to abdominal pores in some fishes.