Distribution.—Ctenophora are found at the surface of nearly all seas, and many of the genera have a cosmopolitan distribution. Some of the Lobata, the Cestoidea, and the Platyctenea are more commonly found in the warmer regions of the world. Pleurobrachia pileus, Bolina infundibulum, Beroe ovata, and B. cucumis occur off the British coast.
Most of the Ctenophora are from 5 to 20 mm. in diameter, but Beroe reaches the length of 90 mm., Eucharis multicornis a height of 250 mm., and Cestus veneris has been found no less than 1½ metres from one extremity to the other.
Ctenophores usually go about in shoals, and in the case of Beroe cucumis and Eucharis multicornis the shoals may be of very great extent. Pleurobrachia pileus of the British coasts is often found at the end of the season (July) as a series of isolated individuals; but in June they occur in small shoals, swimming so close together that they will choke a tow-net in a very short space of time.
CLASS I. TENTACULATA
Ctenophora provided with a pair of tentacles in the larval stages only or in both larval and adult stages.
Order I. Cydippidea.
This order includes a number of spherical or oval Ctenophores, with a pair of tentacles retractile into deep tentacular pits in the adult stage.
Fam. 1. Mertensiidae.—The body is compressed in the transverse plane, and the ribs on the transverse areas are longer than those on the sagittal areas. The family includes the genus Euchlora, which occurs in the Mediterranean and in the northern part of the Atlantic Ocean. In Charistephane there are only two enormous ctenophoral plates in each of the longitudinal tracts. These plates are so broad that they almost meet laterally to form two continuous circlets round the body of the animal. This genus is found in the Mediterranean, but a few specimens have also been obtained in the Atlantic.
In Tinerfe the body is almost cylindrical, and there is a pair of kidney-shaped swellings at the sides of the aboral pole. It has a pale blue colour, and is found in the Guinea and south equatorial currents of the Atlantic Ocean.
The name Mertensia has been given to several forms that are undoubtedly the young stages of genera belonging to the Lobata, but Chun retains the name M. ovum for a species which is very abundant in the Arctic currents of the North Atlantic.