(I) The polyp, when present, is without the strongly developed longitudinal retractor muscles, forming ridges (taeniolae) projecting into the digestive cavity, seen in the scyphistoma or scyphopolyp. (2) The medusa, when' present, has a velum and is hence said to be craspedote; the nervous system forms two continuous rings running above and below the velum; the margin of the umbrella is not lobed (except in Narcomedusae) but entire; there are characteristic differences in the sense-organs (see below, and Scyphomedusae); and gastral filaments (phacellae), subgenital pits, &c., are absent.
The proximal portion forms the retractor muscles of themanubrium, or proboscis, well developed, for example, in Geryonia.
Behind the penis on the same side is the hook-like columella muscle, a development of the retractor muscle of the foot, which clings to the spiral column or columella of the shell (see fig.
Retractor muscle of foot.
K, Retractor muscles.
Only at the very hindermost end does it pass into the so-called retractor-muscle (fig.
This retractor-muscle, indeed, serves to pull back with great rapidity the extruded proboscis, and is aided in its action by the musculature of the head.
In the posterior retractor the longitudinal fibres become united into one bundle, which, as noticed above, is inserted in the wall of the sheath.
But the whole proboscis apparatus can also be, at least partially, withdrawn into the body cavity, and this is effected by two retractor muscles which run from the posterior aspect of the septum to the body wall (fig.
C, Retractor muscles of the pro- i, Longitudinal muscular layer.
Mr, Retractor muscle.
The withdrawal of the extended polypide is effected by the contraction of the retractor muscles (fig.
Besides the circular subumbral muscles, there may be others running in a radial direction, chiefly developed as the longitudinal retractor muscles of the manubrium.
Anterior retractor muscle of the foot.
M, Posterior retractor muscle of the foot.
C, Retractor of the proboscis.
B, buccal mass; m, retractor muscles of the buccal mass; ov, ovary; od, oviduct; i, coils of intestines; ao, aorta; c', left auricle; c, ventricle.
The heart and vascular system are similar to those of the Neomeniomorpha, the only important differences being that the ventricle is nearly free in the pericardial cavity, and that the latter is traversed by the retractor muscles of the gills.
On the inner surface is a layer of peritoneal epithelium, which is frequently ciliated, and at the bases of the retractor muscles is heaped up and modified into the reproductive organs.
The mouth is devoid of armature, and passes without break into the oesophagus; this is surrounded by the retractor muscles, which are inserted into the skin around the mouth, and have their origin in the bodywall, usually about one-third or one-half of the body-length from the anterior end (figs.
The alimentary canal is U-shaped, the dorsal limb of the U terminating in the anus, situated not very far from the level of the origin of the retractor muscles.
Sipunculoids are dioecious, and the ova and spermatozoa are formed from the modified cells lining the body-cavity, which are heaped up into a low ridge running along the line of origin of the retractor muscles.
I, Retractor muscles.
(ix.) Onchnesoma, with 2 species, and (x.) Tylosoma, with z species, have no tentacles, only one brown tube, and only one retractor muscle.
8, Retractor muscle of head.
Each taeniola bears a strongly developed longitudinal muscle-band, stated by Claus and Chun to be developed from the endoderm, like the retractor muscles of the anthopolyp, but by other investigators it is affirmed that each retractor muscle of the scyphistoma arises from the lining of a funnel-shaped ectodermal ingrowth (" Septaltrichter ") growing down from the peristome inside each taeniola, in a manner similar to the infundibular cavities of Lucernaria, which in their turn are homologous with the sub f genital cavities of Scypho l A .` medusae.
Two retractor muscles pass back from the base of the foot to the dorsal side of the shell.