92 MANUAL OF ZOOLOGY. 



and can be entirely retracted if necessary. The upper part of 

 the coenosarc dilates into a small ciliated cavity, from which 

 are given off two tubes, which proceed respectively to the 

 distal and proximal nectocalyces, where they open into the 

 central chamber from which the nectocalycine canals take their 

 rise. The upper portion of this small ciliated cavity is pro- 

 longed proximally into the larger chamber of the " somatocyst." 

 The coenosarc bears polypites, each of which is protected by a 

 delicate glassy " hydrophy Ilium." 



DIVISIONS OF THE CALYCOPHORID^E. (AFTER HUXLEY.) 



Fam, I. Diphydce. Nectocalyces not more than two in number, and of 

 a polygonal shape. Hydroecium of the pi'oximal nectocalyx complete, or 

 closed posteriorly. Hydrophyllia well developed. 



Fam. II. Spharonectidce. Nectocalyces probably not more than two in 

 number ; the proximal nectocalyx spheroidal, with a complete hydroecium. 

 No hydrophyllia (?). 



, Fam. III. Prayidce. Nectocalyces two in number ; hydroecia incomplete 

 and groove-like. Polypites protected by hydrophyllia. 



Fam. IV. Hippopodidce. Nectocalyces numerous; hydroecia incomplete. 

 Polypites not protected by hydrophyllia. 



ORDER II. PHYSOPHORID^E. This second order of the 

 Oceanic Hydrozoa comprises those Siphonophora, in which the 

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

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

 mal extremity of which is modified into a "pneumatophore" and 

 is sometimes provided with "nectocalyces" The polypites have 

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

 coznosarc. " Hydrophyllia" are commonly present. The repro- 

 ductive bodies are developed upon gonoblastidia. 



The coenosarc in the Physophorida, 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 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 w r ith 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 ccenosarc is termed the " pneumatophore," whilst the chiti- 

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

 (fig. 1 8, i). The pneumatocyst is held in position by the 

 reflection of the endoderm of the pneumatophore over it, and 



