COELENTERATA : HYDROZOA. 97 



Oceanic Hydrozoa comprises those Siphonophora in which the 

 hydrosoma consists of several polypites united by a flexible, con- 

 tractile, unbranched or very slightly brancJud ccenosarc, the proxi- 

 mal extremity of which is modified into a " pneumcUophore," and 

 is sometimes provided with " nectocalyces." The polypites have 

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

 the coenosarc. " ITydrophyllia" are cojnmonly present. There- 

 productive bodies are developed upon gonoblastidia. 



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

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

 necessarily elongated, being sometimes spheroidal or discoidal. 

 The proximal end of the ccenosarc " expands into a variously- 

 shaped enlargement, whose walls consist of both ectoderm and 

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

 cation with that of the coenosarc, and, like it, full of the nutri- 

 tive 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 ot 

 the coenosarc is termed the " pneumatophore," whilst the chiti- 

 nous air-sac which it contains is termed the " pneumatocyst " 

 (fig. 18, 1). The pneumatocyst is held in position by the 

 reflection of the endoderm of the pneumatophore over it, and 

 it doubtless acts as a buoy or "float." In the Portuguese 

 man-of-war (Fhysalia) the pneumatocyst communicates with 

 the exterior hy 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 Physophoridce resemble those of the 

 Calycophoridce in shape, but the tentacles have a much more 

 complicated structure, and are sometimes many inches in length, 

 as in Physcdia. The " hydrophyllia " have essentially the same 

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

 the Physophoridiz c&ctaxa peculiar bodies, termed "hydrocysts" 

 or " feelers " (" fuhler" and " taster" of the Germans). These 

 resemble immature polypites in shape, consisting of a pro- 

 longation of both ectoderm and endoderm, usually with a 

 tentacle, and containing a diverticulum of the somatic cavity, 

 the distal extremity being closed, and furnished with numerous 

 large thread-cells. They are looked upon as " organs of pre- 



