NARCOMEDUS-ffi— HYDROCTENA. 



459 



substance continuous with that of the bell, and not of chordate entoderm as in other Narco- 

 medusi There is no peripheral canal-system. Neither marginal tentacles nor marginal 

 sense-organs of any sort. 



In all respects, excepting its apical sense-organ, peculiar structure of the = tentacles and in 



h3S Tnfg^d^r^tlThave not been observed and the specimens found by Dawydoff 



^l^CSES- Hyir^na and C^Wappear tc .me £ be mere^ 

 lelisms none of which indicate a genetic relat.onsh.p With Ctenophorae The on y ex lus.ve ly 

 C enophore-like character of HyJroctena is the gelaunous nature of the axial ores ot ,ts 

 Tentacles, which differ from those of other Narcomedus* in being h.ghly contractile. 



Hydroctena salenskii Dawydoff. 

 HyJrJ£%Wo, m *cx, , 9 o 5 ! Verhandl. Deutsch. Zool. Gesell., , 5 Jahresvers., p. ,,5. 



Immature medusa.-BeM pyriform, + mm. high. No tentacles or sense-organs at bell" 

 m argm but i-tead 2 long, diametrically opposed tentacles anse from deep sockets >n the 

 Sef of bell and project from the exumbrella nearer to apex than to bell-marg n. The ten 

 IZs ar! J l P "e attack lateral filaments, highly contractile, and can be partially withdrawn 



F,c. joj.-ff,*.^ *~*H, -Pied from Dawydoff, in Mte. Acad. Sci. St. Petersburg. A, side view of medusa; 

 303 n.v b ^ saggital section; C, v.ew of apical sense-organ. 



within their pit-like sheaths. Their ectoderm contains nematocysts similar to those of Hydro- 

 m dusi buMheir axial cores do not cons.st of entodermal cells but of the gelatinous ; substance 

 rmesoeTcea) of the bell as in Ctenophor*. This gelatinous substance contain, the muscle- 

 "ls whTch enable the tentacles to contract. Surface of exumbrella smooth and devoid of 

 cuia Ix epting at aboral pole where we find a remarkable sense-organ consisting of a deep 

 c ated I cS-like pit, the exterior opening of which is surrounded by a thick, pigmented 

 ne of ectode mal C ell bearing long, hair-like bristles. The deep-lying end of the pit contains 

 l g sphe ica lnhocysts, each mounted on a slender stalk. These lithocysts are diametrically 

 opposed I ino/clear from Dawydoff's description whether they are in the tentacular 

 dfame er o in a plane 9 o° from the latter, but judging from his figures which we have copied 

 th^Ue in the same plane with the tentacles. The cilia of the canal of the sense-organ are 

 vibratileTn life. There are no excretory pores at the bell-apex near the sense-organ, such as 

 are seen in Ctenophorae. 



