CTENOPHORA. 721 



ciliated. The cilia-plates supporting the otoliths, the bell covering the 

 sensory area, and the ctenophoral plates, are alike formed by the agglutin- 

 ation 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 metamor- 

 phosis. 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 rudi- 

 mentary. 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 forma- 

 tions. 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 Eiicharis 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 development *. 



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 Cteno- 



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 Beroe. See Z. W. Z. xlii. 1885. 



1 This fact has been confirmed by Graeffe : see Arb. Zool. Inst. Wien, v. 1884, p. 362 ( = p. 30 

 of his article). 



3 A 



