102 COMPARATIVE AX ATOMY. 



diminution of these structures. Saphenia has only two tentacles : 

 in some forms, only one is developed (Stenstrupia). In the Trachy- 

 nemidae also, the tentacles are arranged radially, and many, as the 

 ^Eo-midas, have interradial ones in addition. The attachment of the 

 tentacles to the body is peculiar, as the tissue which supports them 

 often sends a considerable process into the body. Reduction occurs 

 here also. ZEginopsis has only two tentacles. In the Geryonidas a 

 change of tentacles takes place, the young animal having filaments 

 which are not permanent (larval tentacles) and which are different 

 in structure from the permanent ones. 



The oral tentacles distributed among the Hydromedusas like- 

 wise correspond in number to the radii of the body. They are 

 sometimes simple, sometimes branched. They are not, however, 

 always present, and are frequently replaced by outward growths of 

 the edge of the mouth. They arc generally wanting in the Trachy- 

 nemidse and ^Eginidas. 



Among the Siphonophora all the medusiform persons want the 

 marginal filaments, which seem to be indicated merely as rudiments ; 

 as, for example, in the nematophorous enlargements of the protective 

 persons. This want of an organ important in the economy of the 

 colony is compensated for by the "tentacles" and the "grappling- 

 lines," which can be shown to be modifications of medusiform 

 persons (§ 75). 



The marginal filaments are wanting in the divisions of the 

 Rhizostomidas and Cyanete among the Discophora; they have four 

 large groups of tentacles which arise from the lower surface of the 

 umbrella, and which can be considered either as marginal filaments, 

 or as oral tentacles. In others there are marginal filaments present, 

 corresponding in number to the radii, and sometimes even interradial 

 ones are present. Even in the Charabdcida?, Charabdea has four 

 tentacles carried by the arrow-shaped processes of the bell; in 

 Tamoya (T. quadrumana) these are represented by the same number 

 of tufts. The filaments are more numerous in the Pelagire, whilst 

 the Aurelias are distinguished by a very large number of fine marginal 

 ones. Oral tentacles are developed as fine fringing processes on 

 the edges of the arms which surround the mouth. In the Rhizo- 

 stomidas they are distributed along the numerous grooves which 

 carry the oral pores. 



Two kinds of marginal filaments may be observed in the Lucern- 

 arias ; in one division (L. cyathif ormis) the filaments which beset the 

 edge of the cup-shaped body are just like those which are found in 

 the Medusa?, but they may be seen to be broken up into eight 

 groups ; in the other (L. auricula) they form eight tufts placed on 

 the ends of the four processes which project from the body. 



The tentacles of the Anthozoa are different in the large groups of 

 that class. Eight lamellar tentacles, indented or feathered in appear- 

 ance, surround the mouth of the Octactinia. There is a much larger 

 number of cylindrical tentacles in the Hexactinia. They surround 



