152 



COELENTERATA. 



The planula develops at one pole a pneumatocyst as a thickening and involu- 

 tion of ectoderm (like the entocodon of a medusa bud), and at the other a siphon 

 (Fig. 127 d). The pneumatocyst is supposed by Chun to be homologous with 

 the primary nectocalyx of the Calyconectae. In some forms the upper part of 

 the body gives rise to a cap-shaped hydrophyllium as well as to a pneumatophore 

 (Fig. 127). The crown of hydrophyllia which is sometimes formed persists only 

 in Athorybia, Avhere nectocalyces are not formed. In Agalmopsis and Physo- 

 phora the primary hydrophyllia of the larva (Fig. 128) fall off as the stem 

 becomes larger, and are replaced by nectocalyces. 



FIG. 127. Development of Agalmopsis Sarsii (after Metschnikoff). a, planula ; b, stage with 

 developing hydrophyllium D; c, stage with cap-shaped hydrophyllium (D)and developing 

 pneumatocyst Lf; d, stage with three hydrophyllia (D, D', D"), siphon P and tentacle; 

 Lf pneumatocyst. 



Fam. 1. Circalidae. Monogastric Physonectae with a corona of nectocalyces, 

 without bracts. Circalia H. 



Fam. 2. Athoridae. Monogastric Physonectae with a corona of bracts, without 

 nectocalyces. Athoria H. ; Athoralia H. 



Fam. 3. Apolemidae. Polygastric Physonectae with a long tubular stem 

 bearing numerous siphons, palpons, and bracts ; each siphon with unbranched 

 tentacle. Nectocalyces biserial ; either two opposite nectocalyces, or two alter- 

 nate series of opposite nectocalyces. Pneumatophore without radial pouches. 

 Nectocalyces with tentacles arising from the stem. 



Sub-fam. 1. Dicymbidae. Two opposite nectocalyces only. Cormidia 

 monogastric ; each with a single cyston. Dicymba H. 



Sub-fam. 2. Apolemopsidae. Two opposite rows of nectocalyces. Cor- 

 midia polygastric ; each with several cystons. Apolemia Esch. ; Apolemopsis 

 Brandt. 



