Invertebrate Gallery of the Indian Museum. 115 



only one gill-plume — the right— functionally developed; 

 (3) the Opisthobranchta, in which the gill-tuft, often 

 uncovered by the mantle, is placed often on the after end 

 of the body and always behind the heart ; and (4) the 

 Pulmonata, in which the free edge of the mantle is fused 

 with the body-wall so as to convert the mantle-chamber 

 into a closed sack, open only in front, which is used as a 

 lung. 



i. GASTROPODA [ANISOPLEURA] ZYGOBRANCHIA. 



[(Ea0«0 I26ar— I29f ]. 



The Zygobranchia are characterised by the possession 

 of a pair of gill-plumes, by the absence of special genital 

 ducts, and by the large simple cap-like shell. The Order 

 includes three families of herbivorous marine littoral mol- 

 lusks, namely, (i) the Haliotidx ox Earshells, (2) the Fis- 

 stirellidas ox Key-hole Limpets, and (3) ihePatellidxox true 

 Limpets : all three are widely distributed, the Ilaliotidse 

 being most numerous in the Australian region, and the 

 Fissurelidse in South America. All three families are well 

 represented in Cases 126C — 129B. The sexes in the 

 Zygobranchia are distinct. In the true Limpets the 

 gill-plumes are rudimentary, and the functional breathing 

 organ is a fringe at the circumference of the mantle — see 

 the preparation above Case 128. 



ii. GASTROPODA [ANISOPLEURA] AZYGOBRANCHIA. 

 [(Eases l29ar-lC6|5]. 



In the A zygobranchia owing to the torsion of the visceral 

 mass to the right the important organs (gill-comb, kidney, 

 etc.) on the left side have atrophied and disappeared 

 The sexes are distinct, and special genital ducts are 

 present. A large spiral shell, often of many whorls, is 

 present, and the mouth of it is often closed by a horny or 

 stony operculum. The Order is a very large one and 

 includes both terrestrial and freshwater as well as a 



H 2 



