Introductioii to Aiii;nal Morphology. 99 



■expanded into a variously shaped pneumatophore, 

 containing at its apex a chitinous air-sac (pneumato- 

 cyst), which does not communicate with the cavity of 

 the pneumatophore, or with the somatic cavity, but 

 opens externally by a pore or sfigriia (except in 

 Physophora, Forskalia, Agalma, Athorybia, and 

 Halistemma), and is held in its place by a reflection 

 of the endoderm. Its shape may be spheroidal 

 (Athorybia), oval (Physophora), cylindrical and small 

 (Agalma), &c. The hydrophyllia and tentacles are as 

 in Calycophoridse ; and there are also, on the ccenosarc, 

 Jiydrocysts, polypoid, protective, or prehensile processes 

 of endo- and ectoderm, each with one long, filiform 

 tentacle, closed distally, and profusely armed with 

 trichocysts. The tentacles may be simple coecal 

 tubes with vacuolated endoderm (Velella), or branched 

 at the end (Porpita). Each may be at the base of a 

 polypite (Apolemia), or projecting from the ccenosarc 

 independently of the polypites, each with a basal sac 

 (Physalia). In Physophora, the tentacle has a large, 

 spheroidal involucre enclosing the sacculus. From 

 the air-sac to the w^allof the pneumatophore, septiform 

 processes pass in Forskalia and Agalma. In Velella, 

 slender, jointed, air-holding processes project from 

 the pneumatocyst into the wall of the hepatic organ 

 (pneumatic filaments). In Rhizophysa, long, branched, 

 cellular processes, covered by ciliated endoderm, pro- 

 ject from the wall of the pneumatocyst into the cavity 

 of the pneumatophore ; perhaps like the hepatic organ 

 of Velella. 



The medusiform buds form on blastostyles, and are 

 often free ; males (androphores) and females (gyno- 

 phores) may co-exist, or be separate. The appendages 

 ■on the ccenosarc are usually unilateral. 



H 2 



