368 NUDTBRANCHIATA. 



JDistr. — 6 sp. Canaries, Mediterranean, India, China, Sand- 

 wich Islands. Fossil, 4 sp. Jurassic — ; United States, Sicily, 

 Asia. U. Mediterranean Lam. (Ixxxix, 87). U. Indica^ Lam. 

 (Ixxxix, 88). 



Shell limpet-like, orbicular, depressed, marked by concentric 

 lines of growth ; apex subcentral, oblique, scarcely raised ; 

 margins acute; inner surface with a central colored and striated 

 disk, surrounded by a continuous irregular muscular impression. 

 It has a minute sinistral nucleus. 



Tylodina, Rafinesque. 



Distr. — 3 sp. Mediterranean, Norway. Fossil, 1 sp. Tertiary. 

 T. jjunctulata, Raf. (Ixxxix, 89). 



Shell limpet-like, depressed, apex subcentral, with a minute 

 spiral nucleus. 



Animal oblong, foot truncated in front, rather pointed behind ; 

 dorsal tentacles ear-like, with eyes sessile at their inner bases ; 

 oral tentacles broad ; branchial plume projecting posteriorly 

 on the right side. 



Order NUDIBRANCHIATA. 



Animal destitute of a shell except in the embryo state ; 

 branchise always external, on the back or sides of the body. 

 Sexes united. 



The Nudibranchiate 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 sea-weed. They have been found in the 

 Icy Sea, at Sitka, and in the sea of Ochotsk; in the tropical and 

 southern seas they are abundant. They require to be watched 

 and drawn whilst living and active, since after immersion in 

 spirits they lose both their form and color. In some the back 

 is covered with a mantle (viii, 39), which contains calcareous 

 spicula of various forms, sometimes so abundant as to form a 

 hard shield-like crust. The dorsal tentacles and gills pass 

 through holes in the mantle somewhat like the " key-hole " in 

 Fissurella. In others there is no trace of a mantle whatever 

 The eyes appear as minute black dots, immersed in the skin, 

 behind the tentacles ; they are well organized and conspicuous 

 in the young, but often invisible in the adult. The dorsal ten- 

 tacles are laminated, like the antennae of many insects ; the}^ are 

 never used as organs of touch, and are supplied with nerves 

 from the olfactory ganglia. The Doi'idida are distinguished by 

 having a short and wide lingual membrane with numerous similar 

 teeth ; the vEolids have a narrow ribbon with a single series of 

 larger teeth. In Dendronotus a lai'ge central tooth is flanked by 

 a few small denticulated teeth. 



