FIROLID^E. 



227 



FAMILY XXXIX. PHYLLIDLD^E. 



In the genera PhyUidia and Diphyttidia the animal is shell-less, the internal organs being 

 covered by the mantle ; the gills form a series on both sides 

 of the body, between the foot and the mantle. They are 

 found in Britain, Norway, and the Red Sea. 



Division b. NUDIBRANCHIATA. The Sea Slugs are found 

 on all coasts where the bottom is firm or rocky, from between 

 tide-marks to a depth of fifty fathoms. A few species are 

 pelagic, crawling on the stems and fronds of floating seaweed. 

 They have been found in the Icy Sea and in the Sea of 

 Okhotsk, whilst in ti-opical and southern seas they are abundant. 

 The animal is destitute of a shell, except in the embryo state. 

 The branchia3 are always external on the back and sides. 



FAMILY XL. DORID^E (SEA-LEMOXS). 

 The gills are plume-like, and placed in a circle in the 

 middle of the back. Genera : Doris, Goniodoris, Triopa, 

 sEgirus, Thecacera, Polycera, Idalia, Ancvla, and Ceratosoma. 



A, IDALIA ; B, MIRANDA ; C, DEXDROXOTUS , 



FAMILY XLI.~TRITONIAD,E. D , DOT o; E, HERM.KA ; F, GLAUCUS. 



The gills in this family are arranged along the sides 



of the back ; the tentacles are retractile into sheaths. Genera : Tritonia, Scyllcea, Tethys, Bornella, 

 Dendronotns, Doto, Melibcea, and Lomanctus. 



FAMILY XLII. ^ 



In this family the tentacles have no sheaths and are non-retractile ; the gills are placed on 

 the sides. Genera : jEolis, Glaucus, Fiona, Embletonia, Proctonotus, Antiopa, Hermcea, Alderia. 



FAMILY XLIII. PHYLLIRHOID^E. 



These are pelagic footless Sea Slugs, swimming with a fin-like tail. They have two dorsal 

 tentacles and no gills. Genus : PhyUirhoe. 



FAMILY XLIV. ELYSIAD^. 



The animal is Slug-like, without distinct mantle or breathing organs ; the surface of the body 

 is ciliated. Genera : Elysia, Acteonia, Cenia, Limapontia. 



ORDER IV. NUCLEOBRANCHIATA. 



This order is so called because the animals contained in it have the respiratory and digestive 

 organs arranged in a sort of nucleus on the posterior part of the back. All the members are pelagic, 

 swimming on the surface of the sea ; still, they are entitled to a place in the class. They swim 

 rapidly by vigorous movements of their fin-like tails or by a fan-shaped ventral fin, and they can 

 adhere to objects by a small sucker placed on the margin of the lattar. 



Genus Firola. 



FAMILY XLV. FIROLID.E. 



The animal is fusiform, with a long slender head ; fin narrow at the base, having 

 a small sucker ; eyes black and distinct. Eight species are known in 

 the Atlantic, Mediterranean, &c. 



Genus Carinaria. The body is large and translucent, head 

 cylindrical, tentacles slender, ventral fin rounded, tail laterally 

 compressed ; the gills are numerous, and covered beneath by 

 a delicate hyaline. The shell is Limpet-shaped, with a sub-spiral 

 apex. "The Heteropoda are very close to the Gasteropoda, and 

 in most modern works on zoology they are associated with them as a 

 sub-class. They are entirely pelagic, and as it is only under peculiar 

 circumstances that one can stop the ship in mid-ocean and hunt for them, they are little known. One 



CAHINAHIA CYMBIUM. 

 IK, the proboscis ; t, tin- tentacles : /, the foot; 

 *, the shell ; g, the gills, or branchiae ; d, the 



