CALYPTR^. SIPHONARIA. SIGARETUS. 



59 



Fig. 74. 



CALYPTR^EA. 



24. The Calyptrwce have in the interior of the 

 shell, in form of a hollow cone, a small plate pro- 

 jecting downwards, which seems to be the com- 

 mencement of a columella, and is embraced in a 

 fold of the abdominal sac ; their branchiae are 

 composed of long slender filaments like hairs. 

 In some of them the plate or lamina adheres to 

 the bottom of the cone, being itself bent into a 

 portion of a cone or of a tube, and descending 

 vertically. 



25. The Siphonaria deserve notice, because 



they show how much the organization of mollusks may vary with- 

 out any very striking difference in the form of their shell. Until 

 recently these animals have been ranged with the patella3, to be 

 described in the sequel, which they resemble in their shell, but 

 differ from them very much in the structure of their most im 

 portant organs ; their branchia is composed of a few leaflets 

 transversely attached to the bottom of a respiratory cavity, 

 which is on the back, and communicat- 

 ing externally by a lateral hole in the 

 mantle; they do not appear to possess 

 tentacles, but have a narrow veil on the 

 head. 



26. The genus Sigaretus (fg. 75) 

 has a flattened shell, with a slight spire, 

 arid a very large aperture, which is 

 Fig. 75. SIGARETUS. concealed in the mantle. 



ORDER OF GASTEROPODA TTJETJLIBRANCHIATA- 



27. Tubulibranch gasteropods are very analogous to the 

 pectinibranch gasteropods, but are distinguished from them by 

 very important differences in the organization of some of their 

 organs, and in the structure of their shell, which is fixed to sub- 

 marine bodies, and is in the form of a more or less irregular tube, 

 the commencement of which only is spiral. It is divided into the 

 genera Vermetus, Magilus, and Siliquaria. 



24. What are the characters of the genus Calyptrse'a? (Calyptrse'a, 

 from the Greek, kaluptra, a hood.) 



25. What are the characters of the genus Siphona'ria ? 



26. What are the characters of the genus Sigare'tus ? 



27. How are the tubu'libranch distinguished from the pecti'nibranch 

 gasteropods^? (Tubu'libranch, from the Latin, tubulus, a little tube, and 

 branchiae, gills). 



