STRUCTURE, ETC, OP CERATA OP NUDIBRANCHS. 49 



and having been in some specimens retracted completely into 

 the body ; but that cannot have been the case, because, in the 

 first place, it is difficult to understand how a system of cccca 

 extending up into the terminal twigs could be completely with- 

 drawn from a densely branched structure like the cerata ; and, 

 in the second place, some of my sections were made from spe- 

 cimens in which the cerata were suddenly cut off from the 

 living animal with a pair of fine scissors, when fully expanded 

 and healthy, in a dish of sea-water, and these showed the same 

 structure when sectiouised as did the other preserved speci- 

 mens. I mention this, here, to show that this conceivable ex- 

 planation of the absence of the cseca, which might occur to 

 other readers, had been foreseen, and found not to be possible. 

 The argument that as Dendronotus belongs to the group 

 Kladohepatica it is very unlikely to be without hepatic caeca in 

 its cerata is worthless, as Bergh has described an Eolid (Bor- 

 nella excepta) which has absolutely no prolongations of the 

 liver into the cerata.^ 



The large cerata of Dendronotus are, then, as we would 

 expect from our previous examination of the smaller similar 

 cerata of Tritonia, prolongations upwards of the mesoderm 

 and ectoderm of the body-wall, and contain no special structures, 

 such as are found in the cerata of Eolis and Do to. 



The upper part of fig. 17 shows a longitudinal section of one 

 of the cerata, and fig. 18 is a drawing of a transverse section. 

 The ectoderm-cells throughout are of moderate size, of low 

 columnar form, and are not difi'erentiated in any part. The 

 mesoderm, which has the same structure as that of the cerata 

 of Ancula and Tritonia, is penetrated by large, irregular 

 spaces, containing blood- corpuscles. These may be called the 

 ceratal sinuses; they are near the centre of the mesoderm, and 

 run in the main longitudinally (see fig. 1 7, c. s.) ; they occasionally 

 branch, and they open into the numerous minute lacunae which 

 exist in the mesoderm here as elsewhere. Fig. 18 shows a 

 transverse section where several branches of the ceratal sinus 

 are present. In fig. 19 also several spaces containing blood- 

 ' See ' Report upon the " Challenger ■"' Nudibranchiata,' p. 41. 



VOL. XXXI, PART I. NEW SER. D 



