430 THE OPISTHOBEANCHIATE 



especially well seen in Candiella plebeia ; wliile in tlie Hololiepatica a 

 series of forms {Idalia Leachii, I. elegans, and I. aspersa) illus- 

 trates conclusively tlie transformation of the anterior portion of 

 the pleuropodium on the one hand into rhinophoral filaments [Ancula 

 cristata), and on the other into an almost complete rhinophoral sheath 

 {Thecacera pennigera). A survey of the group, indeed, leads to the 

 generalisation that wherever the pleuropodia extend into the head 

 region, their anterior extensions either unite in front of the rhino- 

 phores {Idalia, Triopa, JEgirus, Polycera, Goniodoris, Archidoris, 

 Proctonotidse, &c.), or enter into special relations with them {Ancula, 

 Thecacera, Scyllasa, Lomanotus, Dendronotus, &c.) ; and since rhino- 

 phoral sheaths do not occur in the forms with a closed pleuropodium 

 (except in cases like Lamellidoris sparsa and JEgirus punctilucens, 

 where the tuberculate character of the whole body sui'face is also 

 shown in the raised margins of the rhinophoral fossae), it appears 

 probable that rhinophoral sheaths in all cases contain a pleuropodial 

 element. 



The veil of Lomanotus must not be confused with the apparently 

 similar structure existing in (e. g.) Polycera quadrilineata, which is 

 of pleuropodial nature. It is a true '' oral veil,'' strictly homologous 

 on the one hand with that of Lamellidoris, Acanthodoris, and ^girus, 

 with the paired " oral tentacles " of Archidoris, Goniodoris, Triopa, 

 and Ancula (the rudiments of which also exist in Polycera and Idalia), 

 and on the other hand with the oral veil of Emhletonia and the 

 paired " oral tentacles " of the Proctonotidse and ^oUdidse. It is 

 of course homologous with the veil of Tritonia, Dendronotus, and 

 Doto ; but although the veil in these genera has not the form of the 

 pair of elongate tentacles of the majority of Solids, it is so plainly 

 the same morphological structure that Bergh's^ distinction between 

 the two should not be too finely drawn, especially as his term 

 " margo frontalis " {Stirnrand) could be applied with equal correct- 

 ness to the pleuropodial veil of Polycera, with which the former 

 structure has nothing to do. The same objection can be urged 

 against the use of Fischer's^ term " voile frontal." 



The anal papilla is situated on the right side of the body, under 

 the second primary pleuropodial papilla j it is not easily seen in the 

 living animal, but is readily perceived in one which is well preserved 

 as a slight projection. 



The heart could be seen through the integument of these indi- 

 viduals as an oval structure situated at the level of the interspace 

 between the first and second pleuropodial lobes ; in one individual 

 it was observed to beat sixty-five times in the minute. 



' L. c, pp. 4, 49, &c. 



2 Manuel de Conch} liologie, pp. 526, 535, &c. 



