The tentacles are two in number, and originate from a thickened basis which is sunk in a deep pit or tentacle sheath, the aperture of which is turned aborally. Consequently, the basis lies more or less close to the principal axis, in Pleurobrachia below, i. e. orally to, the origin of the funnel. The tentacle vessel, which is excessively short, is not prolonged into the axis of the tentacle, but gives origin to two ampullae which lie in its basis. The main axis of the tentacle is beset with a single series of simple branches. The whole structure is eminently contractile and can be completely withdrawn into its sheath.
The sexual organs are situated at the sides of each meridional vessel, the male on the sides corresponding to the four interradii, the female on those corresponding to the four radii, i. e. on the subtentacular and sub-ventral aspects of the vessels. They take the form of bands coursing along the whole length of each vessel beneath the rows of ctenophoral plates. The genital products pass into the meridional vessels, thence to the funnel, the stomach, and out by the mouth.
The Cydippidae possess the typical structure above given. The aboral pole of the body may be produced into two or four processes situated in the funnel plane.
The Lobatae differ from the typical structure in several respects. The funnel axis is very short compared to the stomachal, and the right and left halves of the body are produced on their oral aspects into large spreading lobes. The two subtentacular rows of plates are short, and at their oral ends are situated the 'auricles,' which are processes of the body, either long and mobile, or short, the base sometimes of great extent reaching to the mouth. The aboral surface of the auricles is beset with fine ctenophoral plates, the movements of which are independent of those of the plates of the subtentacular rows. The central nervous system lies at the bottom of a deep and narrow furrow: the nerves are prolonged between the plates of each ctenophoral row. The mouth is wide, and there is an oral groove extending to the bases of the body-lobes. The interradial vessels spring from the funnel. The meridional vessels form loops on the body-lobes and communicate with the paragastric canals. The tentacle basis, which is of great length, lies close to the oral pole and there is no tentacle sheath. A tentacle furrow extends right and left to the auricles in which are lodged lateral tentacles.
These are numerous and stretch to the right and left in the tentacle furrows in which they are upheld by peculiar bent ciliary plates or hooks, so that their ends only hang down freely. Eucharis alone possesses a simple chief tentacle. The sexual products are formed either in the short vessels which extend on either side from the main meridional vessels beneath each ctenophoral plate (Eticharis, Bolina alata), or in the walls of the vessels between successive ctenophoral plates.
1R. Hertwig (op. cit. infra, pp. 6, 7) found in a specimen of the Cydippid Callianirabialata four excretory pores, of which two at opposite angles, i. e. in corresponding interradii, were smaller than the other two. He observed the pores open from time to time; at the same time a bundle of cilia was protruded.
The Cestidae have a band-like body, immensely elongated in the stomachal plane, and very short in the funnel plane. The subtentacular ctenophoral rows are reduced to a few plates (4-6) at the aboral pole, whilst the sub-ventral rows extend along the aboral margin of the body to its two extremities. The ctenophoral plates are placed obliquely. The interradial vessels originate from the funnel. The sub-tentacular meridional vessels course along the centre of each surface of the body and at its extremities fall into the sub-ventral vessels which follow the aboral margins, bend round the two extremities and unite with two vessels which follow the oral margin and are branches derived from the oral ends of the paragastric canals. An oral furrow extends right and left along the central line of the oral margin of the body. The tentacle basis has a great lateral length and is protected by a tentacle sheath. A tentacular furrow starts right and left from the sheath and reaches to each end of the body.
There is no chief tentacle, but a number of lateral tentacles are lodged in the furrows in which they are supported as in Lobatae, their free ends hanging down into the water 1. The sexual organs are developed in Cestus along the aboral margin of the body in connection with the sub-ventral vessels; in Vexillum they are present only from place to place in pairs, 5-7 to each half of the body.
1R. Hertwig's account of the tentacular apparatus differs from Chun's. The latter traces all the lateral tentacles to the tentacle basis. The former thinks that they are freed periodically from the basis, but that they remain attached to a narrow band of epithelial cells and muscle fibres on the wall of each tentacular furrow, which dies away before it reaches the tentacle basis.
The Beroidae are conical or ovate in shape, a good deal broader in the stomachal than in the funnel plane. Nets cordigera has two large lateral lobes prolonged aborally. Tentacles and tentacular vessels are absent even in development. The central nervous system is freely exposed, i. e. not counter-sunk in the body, and the edges of the elliptical polar plates are produced into branched villiform processes. The mouth is of great size and expansible: the stomach voluminous, and its oral end armed with sabre-shaped cilia-plates which prevent the escape of the prey. There are no stomachal ridges (p. 723): the funnel is small and there is no vertical funnel vessel, but the two excretory vessels spring separately from the funnel, and the blind ampullae are extended to a great length beneath the polar areae. The perradial vessels are abortive; the vessels are large, the paragastric especially so: the latter divide at the oral margin into a right and left horizontal vessel, not united as generally stated into a circular canal. Into these horizontal vessels fall the corresponding meridional vessels. Both the paragastric and meridional vessels give off lateral branches. The latter traverse the mesoglaea in all directions in Beroe ovata.