DISCOXANTHAE. 141 



organisms, with here and there spots of colour (the hepatic cells of 

 the siphon the apex of the pneumatophore, the cnidosacs of the 

 tentacles, etc.). They are mostly pelagic in habit, but some come 

 from the deep sea. 



The ova are large, generally without vitelline membrane, and 

 undergo a complete and regular segmentation. A free-swimming, 

 solid planula is formed. 



There are two main sub-orders* the Disconanthae, in which the primary 

 form is an 8-radiate medusa, the Disconula, which produces biids on the ventral 

 side of its umbrella ; and the Siphonanthac, in which the promorph is a bilateral 

 medusa which produces buds on the ventral side of the base of its manubrium. 



Sub-order 1. Disconanthae. 



The body (coenosome) formal Inj the umbrella of the original octo- 

 radial medusa, which includes a potythalamous pneumatocyst ; the 

 huds arise in concentric rings from the sub-umbrella. Larva octoradial 

 (Disconula). 



This sub-class includes one order. 



Section 1. DISCONECTAE. YELELLIDAE. 



Siphonophora with a permanent primary umbrella, without necto- 

 calyces and bracts. 



The Disconectae are medusae with a large manubrium (Fig. 117, 

 ms), hollow marginal tentacles, and radial canals opening into a 

 circular canal. There is no velum, and the ex-umbrella surface is 

 pitted inwards in the centre to form an ectodermal sac, the pneumato- 

 cyst (Ik}. From the under side there hangs downwards a number of 

 accessory manubria (gni) which bud the gonophores and are called 

 f/onostyles (f/w). The gonostyles open into the radial canals at their 

 basal ends, while distally they may be closed or open. Beneath the 

 pneumatocyst there is a large cellular mass, the centradenia (cd\ or 

 so-called liver. The pneumatocyst opens on the upper surface by 



* Chun, who objects to Haeckel's separation of the Disconanthae from the rest 

 of the class, arranges the Siphonophora as follows : 



Order 1. CALYCOPHOEIDAE. With nectocalyces without pneumatophore. 

 Order 2. PHYSOPHORIDAE. With pneumatophore. 



Sub-order 1. Haplophysae. Physophoridac with unchambercd pneumato- 

 cyst, which is partly lined by gas-secreting ectoderm and is without tracheae. 

 Tribe 1. Physonectae. "With pneumatophore and nectocalyces. 

 Tribe 2. Pneumatophoridae (Physalidae). Without nectocalyces. 

 Sub-order 2. Tracheophysae (Haeckel's Disconectae). With chambered, 

 chitin-lined pneumatocyst, which gives off tracheae to the polyps. Gono- 

 phores set free as mediisae (Chrysomitra). 



