Ill] SIPHONOPHORA 6V 



(i) the Trachymedusae, (ii) the Narcomedusae, (iii) the Siphonophora 

 and (iv) the Hydrocorallinae. These four groups together with 

 Hydra and the Hydromedusae constitute the first primary division 

 of Coelenterata which is termed the Hydrozoa. 



In the first group the eggs appear to develop from the planula 

 stage directly into Medusae, missing out the hydroid stage com- 

 pletely, but there is some evidence to show that a more correct 

 interpretation of what happens would be to say that the egg develops 

 into a modified hydroid which is then converted into a Medusa by the 

 appearance of a web connecting the tentacles. The sense organs are 

 specially modified tentacles which are suspended like minute clubs 

 round the edge of the bell. In the Narcomedusae the planula 

 develops into a reduced hydroid which attaches itself to the inner 

 surface of the bell of a Medusa belonging to a different group. 

 Medusoid buds are produced by this person and set free. The 

 sensory organs are short clubs which are freely exposed and the wide 

 baggy stomach occupies the whole under-surface of the umbrella, 

 whereas in the Trachymedusae the sensory clubs are enclosed in pits 

 (Fig. 27, II) and the stomach is small and suspended from the 

 umbrella by a stalk traversed by the radial canals. The name 

 Trachymedusae (Gr. rpaxvs, rough) is derived from the circumstance 

 that the umbrella is stiffened by numerous ribs of endoderm cells and 

 the edge has a thick rim of ectoderm. The Siphonophora are stocks 

 consisting both of medusoid and hydroid persons which are not 

 attached to any support but which freely swim or float in the sea. 

 In most Siphonophora some of the medusoid persons known as 

 nectocalyces become locomotor organs and by their rapid pulsa- 

 tions not only drive themselves through the sea, but draw after them 

 the rest of the stock much as an engine draws a train of carriages. 

 Some species, however, like the Portuguese Man-of-war, Physalia, 

 have no nectocalyces and float passively about. The popular name 

 of this genus is derived from the shape of the huge air-containing 

 float from which the persons of the colony are suspended. It has 

 been plausibly suggested that the Siphonophora have been derived 

 from planulae which attached themselves to the surface-film of the 

 water instead of to a solid support. The surface-film in consequence 

 of its physical properties acts like an elastic membrane, and in 

 artificial cultures it can often be seen that some planulae of ordinary 

 Hydromedusae do attach themselves to this, and in consequence 

 perish. But if by favourable variations, such as a tendency to cupping 

 of the. base and an inclusion of air-bubbles in the cavity, the stock 



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