l66 THE INVERTEBRATA 



Order SIPHONOPHORA 



The Siphonophora are colonial animals which exhibit the maximum 

 development of polymorphism found in the Coelenterata or indeed 

 in any group of the Animal Kingdom. They are pelagic and each 

 colony originates from a planula which metamorphoses to form a 

 single medusiform individual (Fig. 124B nec.^ which later drops off 

 from the colony), from the exumbrellar side of which springs a 

 coenosarcal tube budding off all the other members of the colony 

 (Fig. 124 B gst. etc.). It usually happens that those which are de- 

 veloped first are needed to buoy up and propel the young colony. 

 Consequently the first individual is either medusiform or else forms 

 an apical float or pneumatophore, the epithelium of which secretes gas 

 (Fig. 124 A/)w., B nee}). There may also be formed from the ectoderm 

 of the first formed individual an oleoeyst containing a drop of oil. The 

 succeeding medusiform individuals resemble the bell of an antho- 

 medusa, with velum, musculature and canal system but lacking the 

 manubrium, and they are called neetoealyees: while the most primi- 

 tive siphonophores have only a single one there may be a series of 

 them. Following these the coenosarc in one type of colony (Fig. 124 A) 

 grows to a great length and buds off at intervals along its length 

 similar assemblages of individuals. Such an assemblage is known as 

 a eormidium, and may consist of (i) a shield-shaped hydrophy Ilium 

 which covers the rest of the cormidium, (2) a gastrozooid resembling 

 the manubrium of a medusa, with a mouth, and a tentacle usually 

 branched, (3) a mouthless individual, the daetylozooid^ with a tentacle 

 usually of great length and provided with strong longitudinal muscles, 

 and (4) a gonozooid (or individual bearing gonophores) which may or 

 may not have a mouth. The gonophores often resemble those found 

 in some of the Gymnoblastea like Tubularia. Such forms as those 

 described above are the genera Halistemma, Diphyes and Muggiaea. 

 In other cases the coenosarc is not a linear stolon but a massive 

 body from which are budded off innumerable cormidia, in which 

 gastrozooids, dactylozooids and gonozooids are all crowded together 

 to form a compact colony. In Physalia (Fig. 125 B), the ** Portuguese 

 man-of-war ", there is an enormous cap-shaped pneumatophore which 

 floats above the surface of the water. There are no neetoealyees, but 

 the colony is borne hither and thither by the wind and countless 

 numbers are cast up on the lee shores. The dactylozooids of Physalia 

 hang suspended from the colony and form a drift net ; when they are 

 touched by a fish the nematocysts discharge and the fish is captured. 

 The tentacles contract and the prey is drawn up until the gastrozooids 

 can reach it. The lips of these are spread out over the surface of the 

 fish until it is enclosed in a sort of bag in which it undergoes the first 



