ZOOGEOGRAPHICAL REMARKS 93 



about 2 mm. in diameter and i mm. in height, and they have 7-8 tentacles, fairly long, capitate, and 

 held in an upright position. In this regard they resemble the young medusa of Cunina octonaria 

 figured by Brooks (1886), but the eight tentacles are all of equal size. The eight marginal lappets are 

 broad, the peronia deep and broad. Each of the lappets has two sensory organs; the sensory clubs are 

 not yet fully developed, but a short otoporpa is distinctly seen above each of them. A broad peripheral 

 canal is present. Velum is well developed, but fairly narrow. The larval proboscis is reduced, and 

 the ventral wall of the stomach is flattened, with a central mouth opening. 



It is difficult to determine the affinity of these larvae. The number of tentacles and lappets may not 

 remain eight. Capitate tentacles may occur in larvae of Cunina {octonaria, according to Brooks 

 1886) as well as in Pegantha {clara, according to Kramp, 1947). Peripheral canals are present in all 

 species of Pegantha and in some species of Cunina, but are absent in Solmaris and Solmissus. The 

 larvae described above probably belong either to Cunina or to Pegantha, but all known species of 

 these genera have their principal occurrence in the upper layers. The present larvae were found 

 attached to the bathypelagic medusa, Pantachogon haeckeli, and I therefore presume that they belong 

 to an unknown deep-sea species. 



Tabular view of the appearance of the medusae when liberated : 



Tentacles 



Host 

 Bougainvillia platygaster 



Rhopalonema velatum 

 Rhopalonema fiinerarium 

 Pantachogon haeckeli 



Number 

 8 



II 



8 

 8 



Shape 

 Filiform 



Filiform 

 Filiform 

 Capitate, upright 



Statocysts 

 per lappet 



Otoporpae 

 Very long 



Short 



Short 

 Short 



Peripheral 

 canals 



Present 



Absent 

 Absent 

 Present 



Umbrella 



With 8 prominent 



keels 

 Smooth 

 Smooth 

 Smooth 



ZOOGEOGRAPHICAL REMARKS 

 The occurrence of medusae may be influenced by various physical factors, but some of these are of 

 importance only under extreme conditions. For example, in coastal waters with a considerable influx 

 of fresh water from the rivers, the salinity of the water is of decisive importance in the distribution 

 of medusae (although its influence may vary with the species), but within the oceanic areas covered 

 by the Discovery Investigations variations in salinity are not great enough to have a significant 

 eflFect. Oxygen content, nitrates and phosphates may indirectly, through the amount of food animals 

 available, determine the abundance of the specimens, but can hardly alter the specific composition 

 of the fauna. Some species of medusae are morphologically adapted to live in the deep water-layers 

 (where the movements of the water are very slight), and are provided either with a very thick gelatinous 

 mesogloea or with a strong musculature. Frequently, however, it is apparently not the depth itself, 

 but the temperature of the water, which prevents certain species from ascending into the upper layers. 

 Thus in the open ocean a dominant factor in the distribution of medusae is the temperature of the 

 water, and for holopelagic species it is often the only factor of importance, although indeed some 

 of these medusae can tolerate a wide range of temperature. For meropelagic neritic forms the 

 configuration of the sea bottom, to which the fixed polyp stages are attached, may also prevent the 

 penetration of some species into areas where the physical conditions might seem suitable to maintain 

 the medusae. 



In a discussion of the distribution of the medusae it is necessary to take into consideration their 

 life-history and their ecological habitat. We must distinguish between meropelagic and holopelagic 

 species, and within each of these groups some species are predominantly epipelagic, others pre- 

 dominantly bathypelagic, although in some cases no sharp line of demarcation can be drawn. 



