SIPHONOPHORA. 139 



repetition and specialization of certain organs which is demanded 

 by the first view. 



The diagram (Fig. 116) shows nearly all the possible parts found 

 in colonies of Siphonanthae, the largest of the two sub-orders <& the 

 Siphonophora. We may briefly enumerate these and consider their 

 relation to the colony on this " medusome " theory of Haeckel. The 

 stem (St) or trunk is the coenosark or coenosome of the colony ; it 

 is the elongated manubrium of the original larval medusoid, and 

 produces by budding all the parts of the colony. Two parts may be 

 distinguished in it an upper part, the nectosome, to which the 

 swimming organs (nectocalyces and pneumatophores) are attached, 

 and a lower part, the siphosome, bearing the nutritive and repro- 

 ductive organs (siphons, palpons, gonophores). All parts are budded 

 off from the same surface of the stem (the so-called ventral median 

 line), their apparent radial disposition in some forms being due to 

 a spiral twisting of the stem.* 



The swimming organs. The nectocalyx (S) is a medusa with 

 canal system and velum but without a manubrium. The pneumato- 

 phore (Pn) is more difficult of interpretation; it may either be 

 regarded as a medusa, in which the umbrella cavity is the air- 

 chamber or pneumatocyst, or it may be simply regarded and this 

 is Haeckel's view as a part of the ex-umbrella region of the original 

 medusoid larva, the ectoderm of which has become invaginated upon 

 the contained enteric system to form the pneumatocyst (Haeckel 

 distinguishes the ectodermal imagination as the pneumatosac, the 

 secreted chitinous lining as the pneumatocyst). The space round 

 the pneumatocyst lined by endoderm is the pericystic space. 



The siphons (P) may be regarded as polyps, or as the manubria 

 of medusoids. The palpons (tasters, hydrocysts, dactylozooids) are 

 mouthless manubria. The tentacles (Sf) are organs of the siphons 

 (see above). The palpacles (Sf) are similar organs of the palpons 

 found in one order. The hydrophyllia (bracts) are the umbrellas 

 of medusae which are cleft on one side, or which have simply 

 lost their umbrella cavity and of which the manubria are either 

 degenerate or slightly shifted as siphons and palpons. In many 

 forms bracts have undergone a large secondary increase. 



Gfonophores (G) are either budded from the stem or from processes 

 of the latter called gonostyles, which may, or may not be, mouthless 

 polyps. 



* The twisting when present takes place in opposite directions in the necto- 

 some and siphosome. 



