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



and these are always so differentiated, that two opposite lateral vessels (right and left) 

 are symmetrical, different from the two sagittal vessels (shorter ventral and longer 

 dorsal canal). The sagittal plane, which divides the body of the nectophore into right 

 and left halves, is therefore precisely marked by the two latter opposite canals. 

 Usually both halves are symmetrical, but sometimes more or less asymmetrical. The 

 velum which surrounds the ostium of the muscular nectosac is usually rather broad and 

 strong. The four radial canals are connected above the insertion of the velum by a 

 constant circular canal. 



Pallial Canals. — Besides the four constant radial canals, there are in many Caly- 

 conectae one or two pallial canals or nutritive vessels of the jelly-substance of the 

 nectophore. An apical pallial canal runs in many Diphyopsidse from the top of the 

 nectosac to the apex of the umbrella. Praya and others have two pallial canals at the 

 ventral side, an ascending and a descending. 



Number and Arrangement of the Nectophores. — The number of the swimming- 

 bells, and their arrangement on the top of the stem, are employed by modern authors to 

 divide this order into three families : — Monophyidae, Diphyidas, and Polyphyidae. The 

 Monophyidae (or Sphaeronectidae) possess a single large nectophore on the top of the 

 stem ; this is smooth and without distinct edges in the true Sphaeronectidae, sharp-edged 

 or pyramidal in the Cymbonectidse. 



The Diphyidae, comprising the great majority of genera and species (among the living 

 Calyconectae), have always two large nectophores on the top of the stem. These are 

 opposite, of nearly equal size and similar form, with rounded exumbrella, in the subfamily 

 Prayidae. They are also of nearly equal size and similar form in the subfamily 

 Diphyopsidae ; but in this group the exumbrella is sharp-edged, pyramidal, and the two 

 nectophores are not opposed, but one placed behind or below the other. The remarkable 

 subfamily Abylidae is distinguished by two nectophores of very unequal size and 

 dissimilar form ; the first or anterior being much smaller than the second or posterior. 

 The form of the two very different nectophores becomes in this group extraordinarily 

 strange and complicated. 



The Polyphyidae (or Hippopodidae) and the closely allied Desmophyidae are dis- 

 tinguished from the other Calyconectae by the possession of a biserial nectosome, similar 

 to that of most Physonectae. There are here at least four to six nectophores (some- 

 times ten to twelve or more), arranged in two opposite series. They have here no definite 

 edges, and are mitriform or reniform in Desmophyes, Hippopodius, and Polyphyes ; 

 whereas they are angular (pentagonal), prismatic, or pyramidal in Vogtia. 



It may be that all the Calyconectae with rounded nectophores represent a natural 

 suborder of this order (Sphaaronectariae), and all the others (with edged or pyramidal 

 nectophoi^es) another suborder (Cymboncctariae). The first suborder (Sphaeronectariae) 

 comprises the Sphasronectidae, Prayidse, Desmophyidae, and Hippopodidae. The second 



