TRANSACTIONS OF SECTION D. 631 



2. There are those which are essentially processus of the body wall, and have 

 no connection with the liver, as in the penera Tritonia, Ancula, and Dendronotus. 



The six common British genera, Doris, Ancula, Tritonia, Dendronotus, Data, 

 and Holis, show very diflerent conditions of the dorsal processes, and form an in- 

 structive series of types. The general anatomy of all these forms is well known,' 

 and many points in the detailed histology have been worked out, but the method 

 of serial sections giving the histological relations of the different parts of the body 

 has apparently not been up to now made use of by writers on the structure of the 

 Nudibrauchiata. 



In Doris there are rhinophores at the anterior and well-developed branchiie at 

 the posterior end of the dorsal surface. There are no cerata or other dorsal pro- 

 cesses. 



In Ancula there are rhinophores, well-developed branchiai, and large but simple 

 unbranched cerata. 



In Tritonia there are no branchite. The rhinophores are large, and there are 

 email branched cerata along the sides of the dorsal surface. 



In Dendronotus there is practically the same condition. Branchiae are absent, 

 but large rhinophores and elaborately branched cerata form the most conspicuous 

 part of the living animal. In the three last-named genera — Anctda, Tritonia, and 

 Dendronotus — the cerata, whether simple or branched, large or small, are merely 

 processes of the body wall and contain no special organs or structures. 



In the genus Doto there are large rhinophores, no true branchiae, but a double 

 row along the back of very large complicated cerata, which contain branched 

 csecal diverticula from the liver. In fact, in this form there is more of the liver 

 in the cerata than in the body proper. 



In Eolis, finally, we have much the same condition as in Doto, except that the 

 processes of the liver in the cerata, although large, are unbranched, and are not 

 csecal, but communicate with the exterior at their extremities. The cerata also 

 contain at their apices cnidophorous sacs, which open on the one hand to the ex- 

 ternal world, and on the other into the extremity of the hepatic diverticulum.'- 



The cerata then of Eolis and Doto are of an entirely distinct nature, and have 

 a veiy different structure from those of Ancula, Tritonia, and Dendronotus. Of 

 the last group those of Dendronotus are by far the most conspicuous and elaborate, 

 and have been generally supposed to contain hepatic caeca like those of Eolis. It 

 has been shown, however, by Mr. Clubb and mj'self, that this is not the case. 

 Serial sections of carefully prepared specimens show that the liver sends up no 

 diverticula into any of the dorsal processes, and that the cerata contain only 

 structures found in other parts of the dorsal and lateral body walls. What have 

 Ijeen probably taken in dissections for hepatic processes are large blood sinuses 

 which branch through the cerata and communicate in the body with the large 

 dorso-lateral veins.^ These ceratal blood sinuses and those of the lateral body 

 walls with which they communicate are at their junction in close proximity to the 

 liver, and might readily, in the absence of serial sections, be supposed to be con- 

 tinuations of that organ, especially as there is usually a considerable deposit of 

 dark-coloured pigment in the connective tissue aroimd the sinuses in the cerata. 



The large -branched dorsal papillae of Dendronotus are merely a further develop- 

 ment of the small processes of the body wall seen in Tritonia. 



In regard to the function of these dorsal papillae in the Nudibrauchiata, in the 

 first place I do not think that in any case they are brancliial. In Ancula and 

 other forms, we find well-developed cerata existing along with true branchiae, and 

 tlie two have a distinct structure, so that although in sections the cerata and the 

 branchiae sometimes overlap and become displaced, one can always distinguish 



' Our knowledge of these and other Nudibrauchiata is very largely due to the 

 admirable investigations of the celebrated Newcastle zoologists, Joshua Alder and 

 Albany Hancock, the authors of the Ray Society Monograph on the British Nudi- 

 branchiata and many other papers. 



' Sec paper by Herdman and Clubb in Proc. Biol. Soc. Liverpool, vol. iii., p. 225, 

 and PI. XII., 1889. ' Loc. eit. 



