EEPORT ON THE DEEP-SEA MEDUSAE. 51 



thinner towards the pointed distal end. Their structure resembles that of the oral 

 styles of the Margelidse (p. 1, PI. I. fig. 5) and of the solid tentacles of the 

 Peganthidse (p. 30, figs. 10, 11, &c.) already described. Each tentacle, therefore, 

 consists of four different layers: (1) a solid cylindrical endodermal axis, formed of a single 

 row of clear coin-shaped chordal cells; (2) a thin but firm and very elastic fulcral plate ; 

 (3) a thin muscular plate composed of parallel longitudinal fibres ; (4) an exodermal 

 epithelium, bearing partly thread cells, partly pigment cells. The latter contain grains 

 of blackish pigment, and are chiefly found on the abaxial or dorsal side of the tentacle, 

 where they form a black longitudinal streak which represents the direct process of the 

 ocelli and the exumbral ribs of pigment. The tentacles, with their basal ocelli, are the 

 only organs of sense found in the Tesserantha, as in all Stauroniedusae ; special auditory 

 clubs, like those of the other Acraspedse, are not present. 



The deep cavity of the umbrella (subumbrella) consists of a lower simple cavity of the 

 corona of the umbrella, whose vertical axis is occupied by the oesophagus (fig. 2, at), and of 

 an upper quadrilocular part divided by four mesenteries into four conical funnel cavities 

 (fig. 6, ii). These mesenteries or mesogonia (fig. 2, wr) are four thin perradial mem- 

 branes, which stretch vertically between the four perradial angles of the base of the 

 stomach and the middle line of the four radial pouches. They serve principally to attach 

 the oesophagus, are cut out like a crescent at the lower free edge, and pass immediately 

 into the tissue of the subumbrella at the upper, rather thinned, basal margin. The 

 mesenteries must be regarded essentially as folds of the subumbrella, whose structure 

 they share. We find them again in a similar form in the Charybdeidse, Tiaridae, and 

 Pectyllidse (PI. IV. fig. 3, ivr ; PI. VIII. fig. 9, wr). The four interradial funnel 

 cavities (" infundibula subumbralia," fig. 6, ii), which are divided by the four mesenteric 

 folds, are conical sacs, opening below into the umbrella cavity, but projecting more or 

 less with their caeca! point into the central gastral cavity ; their aboral extension could 

 not be exactly defined ; they perhaps extend as far as the tseniola are set with filaments, 

 to the beginning of the basal stomach. 



The muscles of the subumbrella are formed by two different systems, which are found 

 more or less modified in all Acraspedse ; a distal system of circular muscular fibres and a 

 proximal system of radial muscular fibres. The first form the typical coronal muscle 

 ("museums coronarius," figs. 2-4, me), a broad octagonal ring on the umbrella margin, 

 whose eight angles are defined by the bases of the eight adradial tentacles. The system 

 of radial or longitudinal muscles is composed of eight triangular deltoid muscles, whose 

 broad base rests on the proximal margin of the coronal muscle. The four perradial 

 deltoid muscles (figs. 3, 4, md') are narrower and longer, and pass above into the 

 mesenteric folds. The four interradial deltoid muscles (figs. 3, 4, md") are broader and 

 shorter, and their truncated point is inserted at the four septal nodes (kn). 



In Tesserantha, as in all Acraspedse, the " gastrovascular " system consists of two 



