LECTURES ON MOLLUSCA. 235 



Tylodina is intermediate between the Umbrellas and the Pleurobranchs. 

 The head is produced and cleft in front; the foot small ; and the shell 

 shaped like Scurria, but membranous, and with a small spiral, sinis- 

 tral apex. This will probably be hereafter detected in the young 

 Umbrellas. 



Family Runctnid2E. 



The Runcinas are tiny Sea-slugs, with gills like Pleurobranchus, 

 and hard gizzards like the Bubbles. The tentacles are flattened into 

 the mantle. They are supposed to have teeth in three series, and to 

 feed on Diatomacece. 



Family DiphyllidiadtE. 



The Phyllidians are curious creatures intermediate between the 

 Tectibranchs and the Nudibranchs. Diphyllidia has gills going round 

 the back two thirds of the body, the plates being folded in front and 

 behind at right angles to each other. The teeth and horny jaws 

 resemble the Bubbles. There is a curious veil in front of the tiny 

 tentacles, resembling a "respirator." 



Family PhyllidiadyE. 



The Fhyllidias have the general aspect of a Cryptocliiton, the gills 

 being arranged all round (except at the head) between the mantle and 

 the foot. They have no jaws or tongues. The lips are small and 

 conical ; and the tentacles on the back can be drawn into pouches. 

 Fryeria has a rough mantle, and the vent is under the mantle at the 

 back. Uyjjobranchiaja has the mantle extended into swimming flaps. 



Order II. NUDIBRANCHIATA. 



The Naked-gilled Crawlers form a large tribe of mollusks, of strange 

 forms and marvelous beauty. They are found in all parts of the world, 

 from the arctic to the torrid zones, wherever theru- is a firm, rocky 

 bottom, or a crop of sea-weed. When first born, they dwell in a little 

 nautiloid shell, with an operculum; and swim freely with a pair of 

 pteropodal fins. Afterwards they drop fins, shell, and operculum, 

 and become sedate Crawlers, breathing by means of exposed gills on 

 the back, which assume various ornamental shapes, and can often be 

 drawn into cavities of the mantle. In some tribes, the skin is coarse 

 and leathery ; while in others this and the various tissues of the body 

 are so delicate and transparent that we may watch the beating of the 

 heart and the digestive processes. The British species have been 

 admirably examined by Alder and Hancock, and illustrated by the 

 Ray Society in one of the most beautiful Memoirs ever published. It 

 is probable that they are equally abundant in other parts of the 

 world; but they have been very little observed. They are extremely 

 timid; and when disturbed they draw themselves up into a mere lump 

 of jelly or tough skin, so that ordinary collectors would pass them by 

 altogether; and even experienced naturalists must live in their neigh- 



