MORPHOLOGY. 11 



2. Tentacula. 



All the Ili/Jrozoa possess more or less numerous filiform appendages, abundantly provided 

 with thread-cells, and subserving purposes of offence or defence, which receive the general 

 name of tentacula, though they differ very widely in structure and place of attachment. 



In the Hijdndce, CorynidcB, and SertuJariada, numerous tentacula are always attached 

 to the body of the polypite itself, and they usually form a circle not very far below the mouth, 

 though sometimes they are scattered irregularly {Coryne, Cordyloplwrci), and sometimes form 

 a double circlet {Tubidaria). The Calycophoridm and most Physojjhorida have single 

 tentacula springing from the base of the gastric division of the polypite, or from the peduncle 

 just on the proximal side of that base. But in the Physcdiada and FdeUida, the tentacles 

 are wholly distinct from the polypite, arising by themselves from the coenosarc. In the 

 LucernariadcB and Medusida, finally, the tentacles are developed from, the margins of the 

 umbrella and from its under surface. 



The tentacles of the Ilydridee, Corynida, Sertidariada, Li/cernariadep, and Mediisida, are 

 always extremely simple in structure, consisting, at any rate primaril.v, of tubular processes of 

 the endoderm and ectoderm, enclosing a diverticulum of the somatic cavity, and sometimes 

 clavate at the ends, or presenting little papillary elevations, but hardly ever branched. The 

 internal cavity is sometimes persistent, but it very commonly becomes almost obliterated 

 by the vacuolar thickening of the endoderm ; and when this has occurred, the tentacles 

 usually appear as if they had a solid cellular axis. 



Besides these ordinary tentacles, certain Sertulariadce possess organs which must be 

 ranged in the same category, though they differ greatly from them in position and in e.xternal 

 appearance. These vcmatopJiores, as they have been termed by Mr. Busk, are csecal processes 

 of the coenosarc, invested by a continuation of its hard cuticular layer, so as to be quite 

 firm and inflexible. The cuticular investment, however, is open at the end, and in the 

 soft substance beneath the opening lie a number of large thread-cells. These bodies are 

 particularly characteristic of the Plumulariada. 



The tentacles of the various genera of the P/iysojjIiorida and CaJycophorida: differ very 

 widely in structure, gradually increashig in complexity as we advance along a series, the 

 lowest term of which is Velella, and the highest P/iysojjhora. 



The tentacles of Vdella, in fact, differ in no essential respect from those of the Hydridce 

 and Sertulariadce ; they are simple csecal processes of the wall of the coenosarc, with a 

 greatly vacuolated endoderm (PI. XI). Those of Porpita have the same fundamenta. 

 structure, but they are branched at the ends. 



The tentacles of Apolemia are described by Leuckart and Gegenbaur^ as simple, tapering, 

 unbranched filaments, which are beset with large thread-cells on one side, and are traversed 

 by a narrow, ciliated canal. One of these tentacles is said to arise from the base of each polypite. 



The tentacles oiPhysalla (PI. X) exhibit an advance on this structure. They arise inde- 

 pendently from the coenosarc, and each is provided at its base with a large, pyriform, saccular 



^ Z. N. K., p. 69 ; Gegenbaur, p. 40. 



