BRITISH NUDIBRANCHIATE MOLLUSCA. 33 



Alliances. At first sight the Nudibranchiate Mollusca appear to be a peculiar and 

 well-marked group, possessing characters that readily distinguish them from all the other 

 orders of Gasteropoda* A closer examination, however, shows that this is not the case. 

 The Nudibranchiata, though displaying in their typical forms a striking and well recognised 

 character, pass so gradually and almost imperceptibly into the neighbouring groups, as to 

 give rise to considerable difference of opinion on the limits of the order, and the genera which 

 should be included in it. In the present work, the order Nudibranchiata is restricted to those 

 forms with well-marked external branchiae which constitute the original Nudibranches of Cuvier. 

 The anatomical researches of later times have, however, shown so great a similarity in the 

 internal structure between these, and a small group of Mollusks, until lately nearly overlooked, 

 and the extent of which is still imperfectly known — our order Pettibranchiata, and the 

 Enterobranches remibranches and Dermobranches of Quatrefages — that an opinion is gaining 

 ground favorable to their constituting an aberrant group of the order Nudibranchiata. The 

 point of union between these two groups is found in the genus Mysia {Actceon of Oken), many 

 of whose characters approach very closely to those of Hermcea. In their external form the 

 principal difference consists in the want of distinct branchiae in Elysia, their place being 

 supplied by the extension of the sides of the back into longitudinal flaps or folds, that, physio- 

 logically considered, may be supposed to represent the branchial papillae fused into a single 

 organ oh each side. The head and tentacles in Mysia are similar to those of Hermcea, and 

 internally we perceive the same character of tongue and digestive organs, including a highly 

 ramified gastro-hepatic apparatus, which, in this genus, is probably carried to its greatest 

 degree of divisibility. The reproductive organs are also similar in both genera, if it may be 

 assumed that they are formed in Hermcea on the type of those in Cattiopcea {Stiliyer), of which 

 there can be little doubt. Mysia has been considered by some authors to be a pulmoniferous 

 Mollusk, but this appears to be a mistake. With the exception of Placobranchus, a genus very 

 imperfectly known, the other Pellibranchs approach less nearly to the true Nudibranchs in 

 their external characters, though they retain considerable resemblance in their internal 

 organisation. Phyllirrhoe, a genus united to the Nudibranchiata on account of its anatomical 

 characters, by M. Souleyet and Dr. Gray, is a remarkably anomalous and aberrant form, 

 possessing scarcely any external appearance of relationship with this order. 



On the other hand, the Nudibranghs show several points of affinity with the Tectibran- 

 chiata and Inferobranchiata of Cuvier, especially with the latter, two genera of which approach 

 very closely to the Nudibranchiate type. These are Phyllidia and Diphyttidia, both included 

 in this order by M. D'Orbigny and Dr. Gray. In the former genus we find a pustulated or 

 corrugated cloak covering the animal, and pierced, as in Boris, for the protrusion of the dorsal 

 tentacles and the anus. Like the Borididce, too, this genus exhibits a character very peculiar 

 among the Mollusca, that of having the skin stiffened with calcareous spicula. The branchiae 

 of Phyllidia, arranged in plaits around the lower margin of the cloak, in the groove between 

 it and the foot, show a relationship, in the character of these organs, to Patella and Chiton. 

 The genus possesses neither jaws nor denticulated tongue. Diphyllidia has the tentacles 



* " Destitute of any covering, and presenting a marked peculiarity in their external branchiae, 

 their appearance is so very different, that every unprejudiced observer cannot but wonder they should, 

 up to this day, have been mixed up with the true gasteropod shell-fish." — Swaiuston, ' Malacology/ 

 page 36. 



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