SIPHONOPHOEA. 



11 



sarc, and proceed to the margin of the bell, where they all 

 open into a circular vessel. The mouth of the bell is also 

 furnished with a delicate ledge, which runs round its circum- 

 ference, and is known as the " veil." The structure, there- 

 fore, of the nectocalyces is very similar to that of an ordinary 

 medusiform gonophore, the chief difference being the absence 

 in the former of the central polypite or 

 manubrium. The nectocalyces are highly 

 muscular, and have the power of alter- 

 nately contracting and dilating, thus driv- 

 ing the whole organism through the water. 

 In Diphyes (Fig. 19), which may be taken 

 as the type of the group, there is a long 

 thread-like trunk or " coenosarc " (c) which 

 bears at intervals minute polypites, each 

 of which is protected by a delicate glassy 

 overlapping plate, termed a " bract." At 

 one extremity of the coenosarc are two 

 large mitre - shaped swimming - bells or 

 nectocalyces (v, «'), by the contractions 

 of which the entire organism is driven 

 through the water. The coenosarc with 

 its polypites can be withdrawn, when 

 necessary, into a kind of chamber be- 

 tween the two swimming - bells ; but 

 when unretracted the organism often 

 attains a length of several inches. Its 

 name is derived from the fact that the 

 two nectocalyces can be separated from 

 the coenosarc by the least touch, and it 

 was for this reason originally supposed 

 to consist of two distinct animals loosely 

 attached to one another. The tentacles 

 are comparatively speaking of great length, 

 and are furnished with lateral branches 

 containing numerous thread-cells. The 

 mouths of the polypites are not pro- 

 vided with a circlet of tentacles, but each has a single long 

 tentacle arising from near its base. The reproductive organs 

 of the CalycophoridoB are in the form of medusiform gono- 

 phores, which are budded from the stalks of the polypites, 

 and which are mostly detached to lead an independent ex- 

 istence. 



The second order of the oceanic Hydrozoa is that of the 



Fig. 19. — Dipluyes appendi- 

 culata, one of the Vnlyco- 

 phoridcE (after Kolliker). 

 w, v'. Swimming-bells ; o 

 Ccenosarc, carrying the 

 polypites, eaeh with its 

 tentacle and protected by 

 a bract. 



