GASTEROPODA 489 



the aperture upwards. The head and foot are withdrawn into the shell 

 and the edges of the mantle approximate to form an almost complete 

 disc filling up the aperture, leaving only a small hole for breathing. 

 They secrete a membrane mostly composed of Ca3(P04)2 {epiphragma). 

 Several such membranes may be found behind each other. In this 

 winter sleep the snail remains for about six months ; respiratory move- 

 ments are carried on slowly and the heart beats sink from about 

 10-13 to 4-6 per minute. The rate of heart beat is closely dependent 

 on the temperature, and at a temperature of 30° C. is from 50 to 60 

 beats per minute. 



Order STREPTONEURA (PROSOBRANCHIATA) 



Gasteropoda which exhibit torsion, nearly always with a shell and an 

 operculum, with a visceral loop twisted in the form of a figure 

 of eight, the mantle cavity opening anteriorly, the ctenidia in 

 front of the heart, and separate sexes. 



Classification 



Suborder Diotocardia (Aspidobranchiata). Strep toneura always 

 with two auricles and sometimes two ctenidia, the ctenidia 

 with two rows of leaflets (aspidobranch), and the genital 

 products discharging to the exterior by means of the right 

 kidney. 



These are divided into two main tribes according to the 

 characters of the radula : 

 Rhipidoglossa possessing a radula composed of rows of 

 numerous narrow teeth diverging like the ribs of a fan. 

 Haliotis, Fissurella, 

 DocoGLOSSA possessing a radula with rows consisting each of a 

 few strong teeth, very long and used for browsing on the 

 algal covering of stones. Patella^ Acmaea. 

 Suborder Monotocardia (Pectinibranchiata). Streptoneura 

 with a single auricle and ctenidium, the ctenidium always with 

 one row of leaflets (pectinib ranch), with a single osphradium 

 resembling an aspidobranch gill, the gonads with separate 

 ducts opening far forward in the mantle cavity and in the 

 male forming a penis. 



These are divided into four tribes, each with a distinct type 

 of radula, of which three are mentioned below : 

 Rachiglossa: predatory animals; radula with not more than 

 three teeth in a row; always with a siphon. Buccinum^ 

 the whelk. Purpura feeds largely on barnacles. Nassa. 



