96 THE VOYAGE OF H.M.S. CHALLENGER. 



The genus Mitrophyes among the Monophyidse (PI. XXVIII.) differs from all other 

 Calyconectse in the remarkable peculiarity that the convex exumbrella of the single 

 nectophore is covered by a large concave scutiform bract, and that the contracted 

 siphosome is retracted into the space between the former and the latter. We may 

 assume that here the primary nectophore of the larva, instead of being lost, is converted 

 into a permanent bract. 



Siphons or Polypites. — Each cormidium of the Calyconectse possesses constantly only 

 a single siphon, and this is the manubrium of the medusome, the modified umbrella of 

 which is the bract. A single long tentacle is always attached to the base of the siphon, 

 between its dorsal side and the concave bract- wall. When the cormidium is detached 

 from the common stem, and represents an independent free Eudoxia or Erssea, it is 

 therefore called a " Monogastric Calyconecta." 



The siphons of all Calyconectae are of similar form, of moderate or small size, very 

 contractile. The four segments or parts of each siphon are usually very distinct, viz., 

 (1) a short peduncle to which is attached the tentacle ; (2) a hemispherical or subspherical 

 basigaster with a thick wall ; (3) an ovate or ellipsoidal stomach ; and (4) a very mobile 

 proboscis with the terminal mouth. (Compare PI. XXVII. fig. 8, &c.) 



The pedicle of the siphon (sp) is usually very short, sometimes rudimentary, and 

 connects as a narrow cylindrical canal the gastral cavity of the former with the common 

 alimentary cavity of the stem. The tentacle arises from it sometimes more proximally, 

 at other times more distally ; sometimes even more from the following part. 



The basigaster (sb), or the bulbous basal portion of the siphon, is usually subspherical 

 or ellipsoidal, and distinguished by a much-thickened exodermal wall ; this is filled with 

 innumerable densely crowded nematocysts. Its outer and inner surfaces bear a vibratile 

 epithelium. Its small ovate cavity is separated from the stomach by a pyloric valve, 

 which can be closed completely. 



The stomach (sin) is ovate or ellipsoidal, with a thin, non-ciliated exoderm ; its 

 entoderm, however, is very thick, glandular, and often exhibits eight or sixteen hepatic 

 stripes (often of a yellow, red, or orange colour), sometimes instead of these hepatic 

 villi ; some of the entoderm cells are usually vacuolate. 



The proboscis (sr) or the distal portion of the siphon, is very contractile and extensile, 

 with a strong muscular plate ; its free distal end opens through the mouth. The very 

 mobile mouth may be expanded in the form of a suctorial disc, circular, or polygonal ; 

 it is often octolobate (more rarely with four, twelve, or sixteen lobes). 



Tentacles. — The capturing filaments or tentacles of the Calyconectse exhibit in all 

 members of this order essentially the same structure, and are very uniform, compared with 

 the manifold and richly varied forms which we find in the similar tentacles of the 

 following order — Physonectse. In all Calyconectse, without exception, each siphon bears 

 at its base a single, tubular, very long and extensile tentacle, which is beset with a single 



