The members of this order are all free-swimming organisms, and they are placed by many amongst the Hydrozoa, from which, however, they appear to be clearly separated by the possession of a differentiated digestive sac, as well as by their analogies with the Actinozoa, and their generally superior degree of organisation.
Pleurobrachia (Cydippe) may be taken as the type of the order, the structure of all being similar to this in essential points. Pleurobrachia (fig. 89) possesses a transparent, colourless, gelatinous, melon-shaped body, or "actinosoma," in which the two poles of the sphere are termed respectively the "oral" and "apical," and the rest of the body constitutes the interpolar region. At the oral pole is the transverse mouth, bounded by lateral, slightly protuberant margins. "Eight meridional bands, or 'ctenophores' bearing the comb - like fringes, or characteristic organs of locomotion, traverse at definite intervals the interpolar region, which they divide into an equal number of lune-like lobes, termed the 'actinomeres;' but this division of the body does not extend into the immediate vicinity of the poles, before reaching which the ctenophores gradually diminish in diameter, each terminating in a point" (Greene). The normal number of the ctenophores is eight (four or twelve in some other forms), and each consists of a band of surface elevated transversely into a number of ridges, to each of which a fringe of cilia is attached, so as to form a comb-like plate. The cilia in the middle of these paddle-like transverse ridges are the longest, and they gradually diminish in length towards the sides, so that the form of each comb is somewhat crescentic. Beside the comb-like groups of vibratile cilia, Pleurobrachia is provided with two very long and flexible tentacular processes, which are fringed on one side with small cirrhi. These filamentous processes arise each from a sac, situated on one of the lateral actinomeres, within which they can be completely and instantaneously retracted at the will of the animal.
Fig. 89. - Adult of Pleurobrachia rhododactyla, in a natural attitude and of the natural size. (After A. Agassiz.) c One of the ctenophores; t One of the tentacles.
The mouth of Pleurobrachia (fig. 90, a) opens into a fusiform, digestive sac, or stomach (b), the lower part of which is provided with brown cells, supposed to discharge the functions of a liver. The stomach opens below into a shorter and wider cavity (c), termed the "funnel," from which two canals diverge in the direction of the vertical axis of the organism, to open at the "apical pole." These canals are known as the "apical canals" (e), and their apertures as the "apical pores." From the funnel two other pairs of canals are given off. Of these, one pair - known as the "paragastric canals" - turns upwards, one running parallel to the digestive sac on each side (d), and "terminating caecally before quite reaching the oral extremity." The second pair of canals (i) - the so-called "radial canals" - branch off from the funnel laterally, each dividing into two, and then again into two, as they proceed towards the periphery of the body. Thus the two " primary " radial canals produce four " secondary " canals (k), and these, in turn, give rise to eight "tertiary" radial canals (/), which finally terminate by opening "at right angles into an equal number of longitudinal vessels, the 'ctenophoral' canals (f), whose course coincides with that of the eight locomotive bands. These canals end caecally both at their oral and apical extremities" (Greene). The whole of this complex canal-system is lined by a ciliated endoderm, and a constant circulation of the included nutrient fluid is thus maintained.
Fig. 90. - Morphology of Ctenophora. 1. Diagrammatic transverse section of Pleurc-brachia. b Digestive cavity ; i i Primary radial canals ; k k Secondary radial canals; / / Tertiary radial canals ; g Tentacle.
2. Longitudinal section of Pleurobrachia. a Mouth ; b Digestive cavity ; c Funnel; d d Paragastric canals ; e e Apical canals; f Ctenophoral canals ; g Tentacle ; h Ctenocyst. (After Greene.)
Immediately within the apical pole is situated a small cyst or vesicle, supposed to be an organ of sense, and termed the "ctenocyst" (h). In structure the "ctenocyst" consists of a spherical vesicle, lined with a ciliated epithelium, and filled with a clear fluid, which contains mineral particles, probably of carbonate of lime. Just beneath the ctenocyst is a cellular mass. which has been described as giving off eight filaments running along the ctenophores, and is generally believed to be a nervous system. Eimer denies that the central ganglionic mass is nervous, but describes a plexiform nervous system. The reproductive organs of Pleurobrachia are in the form of folds, containing either ova or spermatozoa, and situated beneath the endodermal lining of the ctenophoral canals, one on each side.
The embryo Pleurobrachia is at first rudely cylindrical in form, a belt of cilia passing round the middle of its body. This soon breaks up into two lateral groups, which eventually disappear altogether. The primitive ctenophores are four in number, each ultimately breaking up into two.
As regards the homologies between Actinia and Pleurobrachia, the following may be quoted from Professor Greene:
"If now a comparison be made between this nutrient system" (the canal-system of the Ctenophora) "and that of Actinia, the digestive sacs of the two organisms are clearly seen to correspond in form, in relative size, and mode of communication with the somatic cavity. The funnel and apical canals of Pleurobrachia, though more distinctly marked out, are the homologues of those parts of the general cavity, which in Actinia are central in position, and underlie the free end of the digestive sac. So also the paragastric and radial canals may be likened to those lateral portions of the somatic cavity of Actinia which are not included between the mesenteries. Lastly, the ctenophoral canals of Pleurobrachia and the somatic chambers of Actinia appear to be truly homologous, the chief difference between the two forms being, that while in the latter the body-chambers are wide and separated by very thin partitions, they are in Pleurobrachia reduced to the condition of tubes; the mesenteries which intervene becoming very thick and gelatinous, so as to constitute, indeed, the principal bulk of the body." The "apical" canals, again, by which the digestive sac communicates inferiorly with the external medium, may be compared with the perforation which is found in some of the Actinidae (Cerianthus and Peachia) traversing the axis of the base or foot.
The remaining members of the Ctenophora conform in most essential respects with Pleurobrachia, the most important differences being found in the canal-system. For purposes of comparison this system may be divided into four portions as follows: 1. The "axial system," consisting of the mouth, stomach, funnel, and apical canals; 2. The "paraxial system," comprising the paragastric canals; 3. The "radial system," comprising the primary, secondary, and tertiary radial canals; 4. The "ctenophoral system," consisting of the tubes which run underneath the locomotive bands.
In Beroe, which is in other respects very similar to Pleurobrachia, the axial system of canals is the same as we have seen in the latter. The paraxial system, however, consists of two pairs of paragastric canals, which, instead of terminating cae-cally, open into a circular canal which surrounds the mouth. The ctenophoral canals, likewise, open into the oral vessel, instead of terminating caecally as in Pleurobrachia. Lastly, the radial system is not developed, the ctenophoral canals simply curving round towards their apical extremities, and opening into the funnel directly.
Amongst the Beroidae the mouth extends entirely across the oral extremity of the body; hence they have been termed Eurystomata, the term Stenostomata being applied collectively to all the other Ctenophora.
The Beroidae further differ from Pleurobrachia in being destitute of the long tentacular appendages so characteristic of the latter.
In Cestum, or "Venus's Girdle," "elongation takes place to an extraordinary extent at right angles to the direction of the digestive track, a flat, ribbon-shaped body, three or four feet in length, being the result."
The Ctenophora may be divided into the following groups: