6 THE VOYAGE OF H.M.S. CHALLENGER. 



larval tentacle, which in consequence of the ventral umbrellar cleft has been shifted 

 centripetally to the base of the gastral tube. The primary umbrella itself (Proiocodon) 

 has in consequence of this unilateral development become bilateral ; it develops in the 

 Calyconectse (or Calycophoridse) into the primary swimming-bell, in the other three 

 orders (Physonectse, Cystonectse, and Auronectse) into the pneumatophore. The 

 " swimming-bladder " arises hei'e again as a gland-like invagination of the ectoderm, not 

 however centrally in the apex of the exumbrella (as in the Porpitarise), but in an excentric 

 position. The primary siphon (Protosiphon) persists in the monogastric Siphonanths 

 as a single stomachic tube ; while in the polygastric forms it develops into the stem from 

 which all the other persons of the colony proceed by lateral buckling. The whole 

 structure of the Siphonantbse, as well as that of their Siphonula larvae, suggests the 

 closest relationship with the Anthomedusse, and especially with the family of Codonidse. 

 Only in these Anthomedusse do the reproductive elements develop in the entire gastral 

 wall (as in the manubrium of the gonophores in all the Siphonanths), without hint of 

 radial divisions. Among the Codonidae, however, the subfamily Euphysidse (particularly 

 Hybocodon and Amphicodon) is of especial importance. Here only do three of the four 

 primary tentacles of the umbrellar margin disappear, and one alone remains to attain a 

 proportionately greater development. In this way the bilateral modification of the 

 umbrella is determined. This origin of the Siphonanths is also corroborated by the 

 marked tendency of many Anthomedusse to form Medusas by direct budding from the 

 gastral tube (Codonium gemmiferum, Sarsia siphonophora, &c). Since these Euphysidse 

 develop from Tubularian polyps of the genus Corymorpha, the latter are probably to be 

 regarded as the older ancestral forms of the Siphonanthse. 



CORM and CORMIDIA. 



All Siphpnophorse are pre-eminently characterised by the development of a stock 

 (corm or colony), that is to say, of an individual organism which is composed of several 

 polymorphic persons (zooids or "individuals proper"). The laws and modifications of 

 this compositeness or colony-formation have been as yet but slightly investigated, 

 though they are undoubtedly very interesting and important. I distinguish in the 

 first place simple and multiple colonies. The simple colony (Cormus simplex) consists of 

 a single centralised group of persons, as in all Disconanthae and in the monogastric Siphon- 

 anthse. The compound or multiple stock, on the other hand (Cormus compositus), is 

 formed from the union of several individualised groups of persons, or " groups of 

 individuals scattered upon the stem," — the Cormidia. Such colonies are represented by 

 the polygastric Siphonanths. Each cormidium usually represents, in its general composi- 

 tion of several polymorphic persons, a simple stock, and is in most cases monogastric. 

 There are, however, polygastric cormidia, e.g., Apolemia, Salacia, Physalia. In all 



