NUDIBKANCIIIATA.- 



-MOLLUSCA.- 



-TlUTONIA. 



351 



fool which is adapted for walking, and accordingly they 

 are generally found crawling slowly along, either on the 

 rocks near low ■A'ater mark, on the firm rocky bottom at 

 no great depth, or clinging to sea- weeds out at sea. They 

 do not a]ipear to have the power of swimming through 

 the water ; but they are frequently to be seen floating 

 on the surface in an inverted position. The species are 

 lumierous, and are found in ail parts of the world. The 

 British species have been studied with greater care than 

 the exotic, and no fewer than one hundred have been 

 described by Messrs. Alder and Hancock as inhabiting 

 our own coasts. They e.xhibit a very great variety of 

 form, a high state of organization, and an astonishing 

 variety of lively and beautiful colours. The gills being 

 contractile into cavities on the surface of the body, they 

 |iresent in the living state extremely interesting objects 

 of observation, as tliey keep extruding them and with- 

 drawing them at frequent intervals. In some they 

 present the appearance of flowers ; in others they are 

 arborescent or tree like, or feathered like an ostrich 

 plume ; and in others they are disposed in rows of 

 l^apillary tubercles on the sides of the body. The greater 

 Lumber of these Sea-slugs are carnivorous, and appear 

 to be very voracious. They feed chiefly upon zoophytes 

 and sponges; some aflding to their bill of fare the 

 gelatinous Porpitse and Velellse that are found float- 

 ing on the surface of the ocean ; while others again 

 have been seen devouring other Nudibranchs, and 

 even making a repast upon their own spawn. Messrs. 

 Aider and Hancock tell us that large individuals of 

 the Eolis coronata feed on their own species ; and 

 should a small or weak specimen be within their reach, 

 they seize hold of it by any part which may be nearest 

 to them. "The tail, however, is gencrall}' first seized, 

 and fierce and determined is the onset. The devourcr 

 raises and shakes his papilla; in the manner that the 

 porcupine shakes its quills when irritated, and then, 

 hying back the dorsal tentacles and curling up the 

 oval ones, fixes the protruded mouth and jaws upon 

 his prey, when, with a convulsive shrinking up of the 

 body, morsel after morsel is appropriated. In this man- 

 ner it is not uncommon to see an individual entirely 

 devour another, half its own size." These animals 

 de|iosit their ova in coils, long ribands, or spiral chains. 

 The eggs are very numerous. Mr. Darwin notices 

 particularly a species found on the shore of the Falk- 

 land Islands. The ova were contained, from two to 

 five in number, in a spherical little case ; and these 

 were arranged two deep in transverse rows, forming a 

 riband about twenty inches in length. By computa- 

 tion of the number of rows, and the number of little 

 cases in each row, he reckoned that in that riband there 

 could not be fewer than six hundred thousand eggs ! 

 Upon escaping from the ovum the larva, or young, 

 is extremely minute, and is described as resembling 

 a rotiferous animalcule more than a mollusc. It is 

 inclosed in a transparent, calcareous, nautihiid shell, 

 with an operculum. In lieu of the tentacles which are 

 possessed by the adult, this young creature lias two 

 veils, shaped somewhat like cars, and which give it the 

 power of moving rapidly through tlie water. At a later 

 period these veils disappear ; the foot becomes enlarged 

 and the shell falk off; the ereature begins to crawl like 



a Gasteropod, and the gills make their appearance. 

 When the veils fall oft', and the tentacles and gills are 

 fully evolved, the metamorphosis is complete, and the 

 animal assumes its mature state of existence. The 

 various genera which compose this order may be all 

 arranged in two large groups, according to the situation 

 of the gills or branchiie. 



Guoup I.— PYGOBRANCniA (Croy) = A^TIlo- 

 Branchiata (Adams). 



In this group the gills arc plumose or branched, and 

 are placed in a circle or semicircle round the vent, on 

 the middle of the hinder part of the back. The skin 

 ie tough and coriaceous, of a spongy or cellular struc- 

 ture, and stifl'ened with numerous imbedded spicula 

 more or less definitely arranged. In some of the 

 families in this group, as in OnchldurklcE and Don'dw, 

 the body of the animal is of a convex form, the mantle 

 large, reaching to the edge of th-e foot, and destitute 

 of any appendages. The dorsal tentacles are two in 

 number, retractile ; and the mouth tentacles are also 

 two, when present, but are sometimes wanting. These 

 two families are represented on Plate 5 by several 

 species. In all the other families of this group the 

 body of the animal is more or less compressed, the 

 mantle small, exposing the head and foot, or some- 

 times even obsolete, arid often furnished on the margin 

 with beard-like filaments. Such are the families Gonio- 

 doridw, Polyceratida; and Ceratosomidw,iis represented 

 in Plates 5 and 6 by several species. 



Group II.— AIOLOBRAKCIIIATA. 



The second large group of Nudibranchs are all distin- 

 guished from the preceding group, hj' not having the 

 gills arranged round the vent, but on the contrary placed 

 usually in rows along the sides of the body. The exact 

 position and form of these organs vary, however, a good 

 deal. In some (the /»/e>-oi»-a?;67(ra of Gray) they are 

 placed on the under side of the expanded mouth, and 

 are lamellar. Such are the families Phyllidikke and 

 D/pfnjllidiidcv. In another series of this group (the 

 rolyhranchia of Gray) the gills are placed on the 

 upper surface of the mantle, and are either lamellar or 

 plumose. Such are the families Tritoniida:, Scyllaida; 

 and Tctliyida;. 



Genus Tritonia. — The Tritonias are rather nume- 

 rous in species, and many of them are found upon 

 corallines or sea-weed, and under stones, in sliallow 

 water, in which places they feed upon the Zoophytes 

 that live in their neighbourhood. — See Plate G, fig. 1 

 [Tritonin Hombergii). 



THE TRITONIA ARBOEESCENS is remarkable for 

 the sound it produces. AVhen observed in a glass vessel 

 in a room, the sounds are audible at the di.stance of 

 twelve feet, and resemble very much the clink of a steel 

 wire on the side of the jar ; only one stroke is given at 

 a time, and this is repeated at intervals of a minute or 

 two. " The sounds obviously proceed from the mouth 

 of the animal ; and at the instant of the stroke we ob- 

 served the lips suddenly separate, as if to allow the water 

 to rush into a small vacuum formed within." — {Grant.) 



