BIPHORA. 



found in the most ancient fossiliferous strata or layers of the 

 earth's crust. 



25. The Orbicul^ have one round, conical valve, like the 

 shell of the Patellae {fg. 78, p), while the other is flat and 

 perforated for the passage of a very small peduncle ; the confor- 

 mation of their arms, and the arrangement of their branchial 

 vessels, very closely resemble those of the Terebratulee. 



CZiiiSS OF JiHOT^lASSCA. TiriTICATjA., OR ikCEPHAZiA 

 -WITHOUT SHELiLS. 



26. The acephalous mollusks without shells, which have also 

 been called Tunicata, differ much from all the preceding in their 

 general form, as well as in many important particulars of their 

 organization. Their mantle consists sometimes of a simple 

 tube open at both ends, and sometimes of a sack ; their branchiae 

 present difl^erent forms, but always very little developed, and arc 

 never divided into four leaflets or laminae, as in the ordinary 

 acephaloe, or lamellibranch testacea, among which they are 

 placed by many authors ; they have neither foot nor arm ; and 

 they evidently form the connecting link between the mollusks 

 we have just described and inferior animals which are ranged 

 among the Polypi in the branch of Zoophytes. 



27. The BiPHOR^ of all the Tunicata possess the most com- 

 plicated organization. Their mantle is tubular, furnished with 

 transverse muscular bands, and enclosed in a transparent, car- 



hr 



Fig. 114. BIPHORA. 



tilaginous envelope ; both extremities are open, and the posterior 

 orifice (p) is supplied with a little valve, so arranged as to 

 admit the water, but not to allow its escape ; the mouth 



Explanation of Fig. 114— a. the anterior opening of the mantle;— 6. the 

 mouth ;— /. tlie liver, &c. ;— on. the anus \—hr. the branchiae ',—p. posterior 

 opening of the mantle. 



25. What are the characters of the genus Orbicula ? 



26. What are the general characters of the class of Tunicata ? 



27. What are the characters of the genus Biphora ? 



