The branches of the sub-tentacular and sub-ventral vessels of the anterior half of the body unite inter se as well as with the paragastric branches: so too the corresponding branches of the posterior half of the body, but there is no lateral connection between the anterior and posterior networks in B. ovata as there is in B. Forskalii, and more especially in Neis cordigera. Idya has no branches from the paragastric vessels. The sexual products are placed at the sides of the meridional vessels and more or less in their lateral branches, especially in B. Forskalii: in B. ovata and Idya they extend along the canals nearly to the oral marginsl.
The egg of Ctenophora is suspended in a large mass of jelly inclosed by a delicate membrane. As to the development the gastrula is epibolic: and the gastrula mouth is said by Chun to correspond with the aboral pole. The mouth and stomach are formed by an invagination of the ectoderm: the endoderm giving origin to the funnels and the various vessels. The muscular fibres in the mesoglaea are said to be derived from immigrant ectoderm cells both superficial and stomachal2. The ectoderm is at first ciliated. The cilia-plates supporting the otoliths, the bell covering the sensory area, and the ctenophoral plates, are alike formed by the agglutination of cilia. The embryo is hatched at a period before its system of vessels and its other organs have acquired their adult condition. The Lobatae and Cestidae undergo a pronounced post-embryonal metamorphosis. The larva is at first a Cydippid-form resembling an adult Mer-tensia, and its funnel plane is longer than its stomachal. The young Eucharis has two tentacles at each end of the funnel axis, an upper which grows and developes lateral branches, and a lower which remains rudimentary. The lobes begin to make their appearance and the body becomes nearly spherical, and at last elongated in the stomachal plane.
The larval tentacles are lost, and the tentacular apparatus of the adult are new formations. A sac or depression appears above each tentacle basis which grows, and in the adult extends above the level of the sensory area. Such sacs are found in no other Lobatae, and perhaps represent the tentacle sheaths of the Cydippidae. The Mertensia-like larva of Cestus differs from that of Eucharis in the branches of the tentacles which terminate in knobs laden with adhesive cells. It passes through a spherical stage and then into the band-like form of the adult. The larval tentacles are lost. The just hatched larval Eucharis becomes in summer sexually mature. Its sub-ventral vessels are dilated, and the dilatations contain both ova and sperm. The former, though only half the size of ova laid by an adult, undergo a normal development1.
1In Neis cordigera the genital products are confined exclusively to the network of vessels. But von Lendenfeld believes that they are derived from cells of the endoderm (?) of the meridional vessels which withdraw into a layer of sub-epithelial cells present in this Beroid, through which they wander to the spot where they ripen.
2This process of immigration may continue during life, according to Chun (op. cit. infra, p. 197). Metsohnikoff denies the accuracy of Chun's and Kowalewsky's observations. His conclusions, briefly put, are as follows: - (I) The embryonic epiblast forms a ring of cells round the cells of the hypoblast, leaving a pore at each end of the principal axis, the blastopore and pseudo-blastopore. (2) A mass of small 'mesoderm ' cells is formed from the hypoblast cells at the blastopore. (3) A
The Ctenophora are transparent, pelagic, and are widely distributed. The Cestidae, however, do not occur in the northern and temperate seas. The Lobatae move their body-lobes energetically and the Cestidae are capable of serpentine undulations. In both cases the motions are caused by the contractions of the sub-ectodermic musculature (p. 722). The animals also sink and rise in the water, the former especially during the day time. In sinking an escape of fluid from the excretory pores has been noted. But, as a rule, all movement is carried out by the ctenophoral plates. The Cydippid Lampetia Pancerina applies its oral aperture to the surface of the water, or some solid object, and gradually expands the stomach to form a flattened sole upon which it glides along, probably by ciliary action. A similar expansion, but to a very much less marked degree, is observable in other Cydippidae and in the young Beroid. All Ctenophora are carnivorous and feed on various pelagic animals, but especially Crustacea. Beroe, however, preys on its congener Encharis. Phosphorescence is general; its seat is in the vacuolate endoderm cells and the generative products. The ovum, embryo, and larva are especially luminous. Some of the Ctenophora are small in size, e. g.
Hormiphora 3/4 in.: others attain large dimensions, e.g. Encharis multicomis 10 inches or more: Beroe Forskalii nearly the same, and a full-grown Cestus 3-5 feet process half-way between epiboly and emboly now takes place. The mass of mesoderm cells pass up the axis of the gastrula to its aboral pole; at the same time the epiblast cells close up the pseudo-blastopore, and are invaginated at the blastopore to form the future stomach. (4) The mesoderm cells multiply and give origin to (a) mesoglaeal jelly cells, and (b) to cells whence are derived in tentaculate forms the musculature of the tentacles. Metschnikoff observed Callianira, a Cydippe, and a Bero'e. See Z. W. Z. xlii. 1885.
1This fact has been confirmed by Graeffe: see Arb. Zool. Inst. Wien, v. 1884, p. 362 ( = p. 30 of his article).
The ectoderm cells lose for the most part their outlines. But the 'glance' cells which contain a clear substance in clumps, the 'granule' or gland cells, the iridescent cells of Cestus, the cause of the deep blue colour when the animal is irritated, retain their individuality. Pigment cells are sometimes present, e. g. in Beroe. Cilia may occur scattered over the body, over the aboral pole, and in Cestus along the aboral margin of the body, and in the oral furrow. Cells with tactile points or bristles are found in the papillae situated on the body of Eucharis multicomis and Deiopea caloktenota1, on the aboral margin of Cestus.