THE CTENOPHORA 



attention of the earlier zoological circumnavigators, Peron, Lesueur, 

 Quoy, Gaimard, and Chamisso ; and in 1829 Eschscholtz assigned 

 to them the systematic position near the Medusae, which they 

 have retained ever since. After Eschscholtz the Ctenophora were 

 studied by many observers, particularly by Leuckhart, Kolliker, 

 Gegenbauer, Fol, L. Agassiz, and Allman, and lately they have been 

 more closely studied by Kowalevsky, A. Agassiz, Metschnikoff, 

 and especially by Chun, whose monograph, forming the first 

 volume of the Fauna and Flora of the Gulf of Naples, is the 

 standard treatise on the subject. 



The fundamental structure of the Ctenophora may con- 

 veniently be studied in two species, which may be procured in 

 abundance off the English coasts in the spring, summer, and 

 autumn months, Pleurobrachia pileus, Fabr. ( = P. rhododactyla, 

 Agassiz), and Hormiphora plumosa, Agassiz. 



The body is ovoid, and in Hormiphora it tapers somewhat 

 towards one end, on which is placed a wide aperture compressed 

 from side to side ; this is the mouth. At the opposite end of the 

 body is a shallow depression containing a sense organ of char- 

 acteristic structure. The line connecting mouth and sense organ 

 is the chief axis of the body ; the extremity, at which the mouth 

 is placed, is distinguished as the oral pole, the opposite extremity 

 as the aboral or sensory pole. 



The surface of the body is beset with eight meridional rows 

 of modified ectoderm, bearing very long cilia, fused together and 

 so disposed as to form a series of swimming plates called combs 

 or ctenes. The meridional rows are termed ribs or costae, 

 and they divide the body into octants. In both Hormiphora 

 and Pleurobrachia they begin at some little distance from the 

 aboral pole, in Hormiphora they extend downwards over about 

 two-thirds of the body, in Pleurobrachia pileus they reach down- 

 wards nearly to the mouth. On either side of the body, in 

 an interspace between two costae, is a pouch leading into a 

 considerable cavity hollowed out in the gelatinous body. From 

 each pouch projects a tentacle, a long solid filament furnished with 

 numerous accessory filaments. 



The mass of the body is composed of a gelatinous substance, 

 so transparent that the main features of the internal anatomy 

 may be studied without dissection. The mouth leads into a 

 tolerably spacious sac which, like the mouth itself, is compressed 

 from side to side. This sac, usually called the stomach, is developed 

 as a secondary invagination of the epiblast, and is therefore a 

 stomodaeum. It extends upwards for some two-thirds of the way 

 towards the aboral pole, and there opens by a small orifice into a 

 second sac, the infundibulum, which is also compressed from side to 

 side, but in a plane at right angles to the first. Following Claus's 



