62 



CTENOPHORA. 



--r.m. 



indicated. But even if these young stages really belong to M. ovum 1 ), they cannot prove that the 

 short tentacle-basis is characteristic of the grown Mertensia. On the contrary the tentacle-basis is 

 unusually long, as I can show. 



In fig. ii is represented the tentacle bases and the central part of the gastrovascular system 

 of Mertensia ovum, drawn from one of the specimens in the "Ingolf" material. It shows the tentacle 



bases as rather long 2 ), characteristically curved 

 bodies, from the upper end of which the ten- 

 t tacle proceeds; this shape the tentacle bases 



have in all the specimens available, and it is 

 likewise seen in the sketch made from a living 

 specimen byVanhoffen (Nordisches Plankton, 

 fig. i); also in Beroe compressa of Mertens, 

 which is, in my opinion, undoubtedly a syno- 

 nym of M. ovum, they have the same shape 

 and size. It may then possibly be right that 

 the tentacle bases are short in the very young 

 specimens, as shown in the two figures of 

 A. Agassiz but in the larger specimens, 

 from a size of ca. 3 mm , they are not "limited 

 to the abactinal part of the spherosome", as 

 maintained by Agassiz. His fig. 29 is too 

 little detailed for showing anything clearly of 

 the shape and size of the tentacle-bases. 



The peculiar feature of the pharyngeal 

 vessels following close along the tentacle bases 



rests on a misunderstanding. The pharyngeal vessels join the pharynx as in other Ctenophores, 

 as I have been able to see clearly both on sections and on the not sectioned material. What Agassiz 

 has taken to be the pharyngeal ("lateral") vessel is evidently the tentacular vessel. 



Moser (Op. cit.) states that the tentacle bases, judging from the figures of Agassiz, are not 

 longitudinally divided as in other Cydippids. This would appear to be really so, also on the preserved 

 material; in sections it is seen, however, that the structure is in accordance with the usual type, only 

 the two halves of the basis lie so close together, partly even overlapping one another, that it can 

 only be seen in sections that there are really two parts as usual. It may be remarked that along 

 each side of the tentacle basis is seen a rather conspicuous series of pigment spots (fig. n); this 

 would appear to be what Mertens mistook for the ovaries. 



') It is peculiar that Fewkes does not mention Agassiz' description of the young Mertensia in his short note on 

 Mertensia ovum (On certain Medusae from New England. Studies from the Newport Marine Zoological Laboratory. Bull. Mus. 

 Comp. Zool. Vol. XIII. 1887. p. 212), where he records that he has traced the development of some Ctenophoran eggs "into young 

 Mertensiae, (which) are possibly of this species". 



2 ) The oral end of the pharynx being destroyed in the specimens preserved it cannot be seen there how far down 

 towards the mouth the tentacle basis reaches; from Vanhoffen's sketch (fig. l) of a living specimen they are seen to reach 

 nearly to the mouth. Is is worth mentioning that in a specimen, which can scarcely have been more than 3 mm long, the 

 tentacle bases have already the same shape and relative length as in the grown specimens. 



l.m ph.v. ph. ph.v m. 



Fig. ii. Pharynx and tentacle-bases of Mertensia ovum. d. diaphragm; 

 inf. infundibulum ; 1. m. longitudinal muscles; oe. oesophagus; 

 p. series of pigment spots; ph. pharynx; ph. f. pharyngeal (stomodaeal) 

 folds; ph. v. pharyngeal vessel; r. m. radial muscles; t. tentacle; 

 t b. tentacle-basis. 



