82 MANUAL OF ZOOLOGY. 



Fam. III. Prayid(e. Nectocalyces two in number ; hydrcecia incomplete 

 and groove-like. Polypites protected by hydrophyllia. 



Fam. IV. Hippopodidcs. Nectocalyces numerous ; hydrcecia incomplete. 

 Polypites not protected by hydrophyllia. 



ORDER II. PHYSOPHORIDJ;. This second order of the Oceanic 

 Hydrozoa comprises those Siphonophora, in which the liydro- 

 soma consists of several poly pites united by a flexible, contractile, 

 unbranched or very slightly branched coenosarc, the proximal 

 extremity of which is modified into a ' pneumatophore,' and is 

 sometimes provided with ' nectocalyces.' The poly pites have 

 either a single basal tentacle, or the tentacles arise directly from 

 the coenosarc. 'Hydrophyllia' are commonly present. The 

 reproductive organs are developed upon gonoblastidia. 



The ccenosarc in the Physophoridce, like that of the Calyco- 

 phoridce, is perfectly flexible and contractile ; but it is not 

 necessarily elongated, being sometimes spheroidal or discoidal. 

 The proximal end of the coenosarc ' expands into a variously 

 shaped enlargement, whose walls consist of both ectoderm 

 and endoderm, and which encloses a wide cavity in free com- 

 munication with that of the coenosarc, and, like it, full of the 

 nutritive fluid. From the distal end, or apex, of this cavity 

 depends a sac, variously shaped, but always with tough, 

 strong, and elastic walls, composed of a substance which is 

 stated to be similar to chitine in composition, and more or 

 less completely filled with air.' (Huxley.) The large proximal 

 dilatation of the coenosarc is termed the ' pneumatophore,' 

 whilst the chitinous air- sac which it contains is termed the 

 'pneumatocyst' (fig. 19, 1). The pneumatocyst is held in 

 position by the reflection of the endoderm of the pneumato- 

 phore over it, and it doubtless acts as a buoy or ' float.' In 

 the Portuguese man-of-war (Physalia) the pneumatocyst 

 communicates with the exterior by means of an aperture in 

 the ectoderm of the pneumatophore. In Velella and Porpita, 

 the pneumatocyst communicates with the exterior by means 

 of several openings called ' stigmata ; ' and from its distal 

 surface depend numerous slender processes, containing air, 

 and known as ' pneumatic filaments.' 



The polypites of the Physoplwridce resemble those of the 

 Calycophoridce in shape, but the tentacles have a much more 

 complicated structure, and are sometimes many feet in length, 

 as in Physalia. The ' hydrophyllia' have essentially the same 

 structure as those of the former order. There occur also in 

 the Physophoridce certain peculiar bodies, termed ' hydrocysts' 

 or 'feelers' ('fiihler' and 'taster' of the Germans). These 

 resemble immature polypites in shape, consisting of a pro- 

 longation of both ectoderm and endoderm, usually with a 



