NUDIBRANCHIATA ; TRITONIAD^l ; 



389 



near low-water mark, where they feed principally on Zoophytes. 



Some of them are more than three 

 inches in length. The TRITO- 

 NIADJE, which are nearly allied to 

 the preceding, have the branchiae 

 arranged along the sides of the 

 back; the tentacles are retractile 



within distinct sheaths, and ofte/i accompanied by branched fila- 



FIQ. 679. TRITONIA HOMBEB.GII. 



FIG. 680. TETHYS FIMBBIATA, TOPER AND UNDER SIDE. 



ments (Fig. 679). To this family also belongs the Tethys (Fig. 

 680) already referred to, in 

 which the head is concealed by 

 a large membranous fringed 

 hood. This animal, which is 

 the largest of the Nudibran- 

 chiata, is said to feed upon other 

 Mollusks and Crustacea. The 

 MQIADM also have the gills 

 placed on the sides of the back, 

 but the tentacles are not retrac- 

 tile. They are active animals, 



and swim freely on their backs. In the jEolis (Fig. 654), the gills 

 consist of an immense number of finger-like processes, forming 

 tufts on each side of the body ; some of these receive ca3cal pro- 

 VOL. ii. E E 



