SIPHONOPHORA 145 



individuals. Such an assemblage is known as a cormidium, and may 

 consist of (i) a shield-shaped hydrophyllium 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 mouth- 

 less individual, the dactylozooid^ 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 Gymno- 

 blastea 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, gastrozooids, 

 dactylozooids and gonozooids, all being crowded together to form a 

 compact colony. In Physalia (Fig. 122 B), the "Portuguese man- 

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

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

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

 numbers are cast up on 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 gastro- 

 zooids 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 stage of its digestion. Physalia can catch and devour a full- 

 grown mackerel, and the poison of its nematocysts is so virulent as 

 to endanger human life. In Velella (Fig. 122 A) the disc-shaped 

 colony has a superficial resemblance to a single medusa. The pneuma- 

 tophore consists of a chitinous disc containing a number of chambers 

 and raised into a vertical ridge which forms a sail. On the under 

 surface there is a single large gastrozooid in the centre, a larger 

 number of gonozooids surrounding it and a fringe of dactylozooids at 

 the margin. The gonozooids produce buds which actually escape as 

 free medusae. The coenosarc consists of a mass of tissue which is 

 traversed by endodermal tubes placing in communication the cavities 

 of the gastrozooid and the gonozooids, and ectodermal tubes 

 (tracheae) which are prolongations of the gas cavity of the pneuma- 

 tophore. This tropical form is often brought in large numbers to the 

 shores of Devon and Cornwall by the Gulf Stream. 



The medusae and nectocalyces of the Siphonophora are very 

 similar to the Anthomedusae from which they may have sprung. 

 Medusae like Sarsia (Fig. 121 C) may bud off other medusae either 

 from the bell or the manubrium, but the Siphonophora are probably 

 not to be regarded simply as a colony of medusae connected by 

 coenosarc. A further change has gone on in which organs have been 



