BRITISH NUDIBRANCHIATE MOLLUSCA. 11 



points in their anatomy and physiology have been elucidated by recent investigations. We 

 shall now proceed to give a more detailed account of the present state of our knowledge on 

 the subject. 



Anatomy. The Nudibranchiata exhibit a high state of organisation, not much inferior 

 to that of any of the Gasteropods ; and as, perhaps, in no other Molluscan group has the ana- 

 tomy been so perfectly investigated, we feel ourselves in a position to take a comprehensive 

 view of their structure, at least so far as the British forms are concerned. Our references to 

 foreign species must necessarily be very limited ; as comparatively few of them have been dis- 

 sected, and even their external characters are, in too many instances, very imperfectly known. 

 Fortunately, however, the British list comprises types of all the larger groups and of a great 

 variety of the smaller ones, so that we are enabled, from our indigenous species alone, to attain 

 a very accurate knowledge of the organisation of the whole order. 



Alimentary System. All the Nudibranchs are provided with a powerful muscular buccal 

 apparatus, which has, in some instances, appended to it a gizzard, as in Lamellidoris, Gonio- 

 doris, Idalia, and others. The oral aperture is always guarded by fleshy lips, and the. mouth 

 itself is furnished with a tongue, bearing a spiny prehensile membrane, and occasionally with 

 lateral corneous jaws. Tethys is the only exception, and in it there are neither jaws nor tongue ; 

 neither is the buccal organ so muscular nor so distinctly defined as usual. The jaws are highly 

 developed in the Tritoniadce and the Eolididce; in the former they are always present, in the 

 latter sometimes wanting, as is the case in Doto, Hermcea, Stiliyer^ and Alderia. A few of the 

 Polycerince are also furnished with lateral jaws ; but they are small, and do not appear to be 

 very efficient cutting instruments. JEgirus has an upper corneous jaw or tooth resembling 

 that of Limax ; and a minute rudimentary under jaw may be detected in some of the Dorides. 



The tongue is composed of a muscular apparatus bearing a stiffish membrane, furnished 

 with small teeth or spines. These are divided into two kinds, central and lateral, distinguished 

 by their position, and generally, when both are present, by a difference in form. The former 

 have been called denies by Professor Loven, the latter uncini ; and the portions of the tongue 

 on which they are placed, are distinguished by that naturalist, under the names of rliaclds and 

 pleura respectively. We have not thought it necessary to preserve these distinctive appella- 

 tions. In the Doridida, the lateral spines are always developed, the central only occasionally. 

 Both kinds are present in the Tritoniadce. The lingual membrane in the typical Dorides, and 

 in the Tritoniadce, is very broad, and is supplied with numerous spines ; it is narrow in Lamel- 

 lidoris and Acanthodoris, there being very few spines in each transverse row, — in some of the 

 species as few as four. In such, the whole of the lingual spines do not amount to more than 

 112, while in Doris tuber culata there are no less than 6,000, and in Tritonia Homberyii, 

 upwards of 36,000. In the Polycerince, the lingual membrane is mostly narrow, and devoid of 

 central spines, and with one, two, or three, large, recurved spines on each side next the median 

 line. External to these, there are generally a few depressed plates or rudimentary spines. 

 JEgirus and Ceratosoma? are, however, exceptions, for in them the spiniferous membrane is broad, 

 bearing numerous spines, similar to those of Doris. 



The lingual membrane in the typical Eolididai is very narrow, being furnished with a 

 longitudinal series of central plates or spines. In the sections of Eolis represented by E. tri- 

 color and E. rufibranchialis, however, there is on each side a single additional lateral spine, 



