REPORT ON THE DEEP-SEA MEDUSAE. xlix 



single row, and are regularly distributed in it according to number and arrangement 

 (§ 77). More rarely two or more rows of tentacles are placed on tbe umbrella margin, tbe 

 one above the other, and are then usually compacted in larger numbers (Pectyllis, Pis. 

 III., IV. ; Pedis, Pis. V., VI.). The tentacles sometimes appeared grouped together in 

 bushes or bundles on the umbrella margin, as in the Lizusidae and Hippocrenidae (System, 

 taf. v., vi.), and in Pectanthis (Pis. VII., VIII.) among the Craspedotae ; and in the 

 Lucernaridae (Pis. XVI., XVII.) and the Chirodropidae (System, taf. xxvi.) among the 

 Acraspedae. Deviation from the original marginal insertion sometimes takes place, as the 

 tentacles either migrate outwardly on the dorsal surface of the umbrella or inwardly on 

 the ventral surface. Exumbral insertion, on the dorsal surface is found in many 

 Trachomedusae and most Narcomedusse (Pis. IX. -XIV.) ; there the tentacles may be 

 placed far up on the exumbrella, but usually denote their original connection with the 

 umbrella margin by the urticating streaks or umbrella clasps already mentioned 

 (" peronia," § 68 ; Pis. IX., XIII. , XIV., en). In the Aurelidse the tentacles are also 

 inserted dorsally (System, taf. xxxii. fig. 8). The Sthenonidae and Cyaneidae are distin- 

 guished by subumbral insertion of the tentacles ; in the latter they are scattered over 

 nearly the whole subumbrella (Drymonema, Pis. XXX., XXXL). 



§,77. Number and position of the tentacles. Although the tentacles of the Medusae 

 present the most varied conditions both as to number and position, still by critical 

 comparison we are able to recognise the existence of certain simple primary and original 

 conditions, from which all the others may be secondarily derived. We may conclude from 

 this that most probably four perradial tentacles (at the distal end of the four radial 

 canals) represent the primitive formation for the Craspedotas, but eight principal tentacles 

 (four perradial and four interradial) for the Acraspedse. In the section of the Craspedotae 

 tetranemal forms (with four perradial tentacles) are found in all four orders ; Codonium, 

 Cytceis, &c, among the Anthomedusae, Tetranema, Eucopium, &c, among the Lepto- 

 medus£e, Petasus among the Trachomedusae, Cunantha among the Narcomedusae (comp. 

 System, p. 359); the two latter may, however, be regarded as already octonemal as in 

 them four interradial cordyli alternate with the four perradial tentacles, the cordyli 

 themselves being merely modified acoustic tentacles (§ 84, comp. PI. IX.). This is also 

 the case in Pericolpa, one of the oldest and simplest forms among the Acraspedae. The 

 inverted condition is shown in CJmrybdea (PI. XXVI. where the four sense clubs are 

 placed perradially, but the four tentacles mterradially. Both the Charybdeidas (Cubo- 

 medusae) and the Pericolpidae (Peromedusae) are derivable from Tessera, the oldest 

 and simplest form among the Stauromedusae, which may at the same time be considered 

 the hypothetic ancestral form of all Acraspedae. This has already eight principal 

 tentacles (four perradial and four interradial). In Pericolpa only the four interridal 

 tentacles are transformed into sense clubs, in Cliarybdea the four perradial, in Ephyra 

 (the ancestral form of the Discomedusae) all the eight principal tentacles. In the latter, 



(ZOOL. CHALL. EXP. — PART XII. — 1881.) M g 



