EMARGINULA. PATELLA. 



61 



passage to the air necessary for respiration, and also for the ex- 

 pulsion of the excrements. 



31. We give the name of Emarginula to mollusks very much 

 like the Fissurellse, but their shell, m 1 place of being open at the 

 top, presents a slit or notch in front, communicating with the 

 branchial cavity. 



ORDER OF GASTEROPODA CYCLOERAI'TCHIATA. 



32. The cyclobranch gasteropods closely approximate the pre- 

 ceding, either in their general form, or in the disposition of their 

 internal organs ; but they are distinguished by their branchiae 

 being fixed around the edges of the mantle (Jig. 78). 



38. Some, the Patella 

 (fg. 78), for example, are 

 provided with a broad-based, 

 conical shell, which covers 

 the whole body ; they very 

 much resemble the Fissurel- 

 lae and Emarginulse, but the 

 shell is not perforated at the 

 apex, nor is there a notch in j i 

 front as in the latter. The 

 head is furnished with two 

 pointed tentacles, having the 

 eyes at their base, and a 

 stout trunk ; the anus is 

 situate to the right, and a 

 little above the head, and the 

 branchiae are composed of 



Fig. 78. PATELLA. 



leaflets or fringes, arranged in a row around the body, under the 

 edges of the mantle. Patellse are found in almost all parts of the 

 world ; they are found on rocks to which they adhere, and lie 

 completely concealed beneath their shell, even after they are left 

 by the ebbing sea. 



Explanation of Fiy. 78. The animal of the patella seen from below ;e. 

 the edge of the shell ; p. the foot ; m. the mantle ; b. the branchiee ; t. 

 the "head. 



31. How does the genus Emargi'nula differ from Fissure'lla? (Emargi- 

 nula, from the Latin, e, from, and margo, marginis, the margin from 

 the margin notched.) 



32. How are cy'clobranch ga'steropods distinguished ? (Cyclobranch, 

 from the Greek, kuklos, a wheel, and bragchia, gills.) 



33. What are the characters of the genus Pate'lla? (Patella, Latin, 

 knee-pan. 



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