There are two tentacles to each inter-mesenteric space, one being marginal and the other circumoral. The gonads are borne upon alternate mesenteries, and both ova and spermatozoa are produced by the same individual.
Fig. 179.—Cerianthus membranaceus. Colour pink, with tentacles annulated pink and brown. About 35 cm. in length. (After Andres.)
The ectoderm of Cerianthus is remarkable for the immense number of nematocysts and gland cells. The latter secrete a quantity of mucus which binds the threads of the discharged nematocysts into a sticky feltwork and this secures particles of sand and mud, the whole forming a long tube in which the animal freely moves. This tube is often of considerable thickness. It is tough and resistant, smooth inside but ragged and muddy outside. It is often many times the length of the animal's body.
The embryo of Cerianthus is set free before the completion of segmentation, and it gives rise to a floating pelagic larva known as Arachnactis. It has a variable number of tentacles and mesenteries according to its age, but when it reaches a size of about 15 mm. in length it has developed characters which are sufficient to determine its position as a Cerianthid.
The genus Cerianthus appears to be widely distributed. C. membranaceus is the common species in the Mediterranean Sea, but a smaller species has been described from Naples under the name C. oligopodus by Cerfontaine. C. americanus occurs on the eastern coasts of North America. The British and North European species is C. lloydii, but another species, C. vogti, has been found at a depth of 498 fathoms in the North Sea. C. nobilis is a gigantic species supposed to be about 1 foot in length when complete, from Torres Straits.
C. bathymetricus of Moseley, placed by Andres in the genus Bathyanthus, is a species of small size (25 mm.), obtained by the "Challenger" from a depth of 2750 fathoms in the North Atlantic. It exhibits a remarkable prolongation of the stomodaeum into the coelenteron in the form of a sack which contained food. Moseley described a species of Cerianthus, 6 inches long, living on the coral reef at Zebu in the Philippines fully expanded in the tropical sunshine.
Several species of Arachnactis larvae have been described. Of these Arachnactis lloydii appears to be undoubtedly the larva of C. lloydii. The adult forms of Arachnactis albida from various stations in the Atlantic Ocean and of Arachnactis americana are not known. The larva of Cerianthus membranaceus has been called Dianthea nobilis, and is characterised by the great length of the column, by the general opacity of all parts of the body, and by the precocious appearance of the median marginal tentacle. A considerable number of remarkable pelagic larvae have been described by van Beneden[[426]] from the Atlantic Ocean, and provisionally assigned by him to five different genera. The adult forms of these larvae are not known, but they are probably members of this order.
CHAPTER XV
CTENOPHORA