HABITS AND PROPERTIES. 49 



tude of the species producing them. They are nearly 

 globular, one axis being somewhat longer than the other, 

 white and opaque. They consist, in general, of an ex- 

 ternal, semi-calcareous, elastic membrane investing the 

 ^Yhole, the interior surface of which is usually studded 

 with numerous rhombic, micros4i)pic crystals of carbonate 

 of lime, some species however having a hard enveloping 

 calcareous shell, of the consistence of that of a bird's 

 egg ; of an inner thin, transparent, shinuig membrane 

 which immediately encloses a transparent and somewhat 

 viscid fluid, analogous to the albumen of bird's eggs ; of 

 the albumen itself, and of the vitellus, which, possessing 

 the same degree of transparency as the albiuuen, cannot 

 be distinguished from it at this time. The elastic eggs 

 when iii'st laid are often flaccid, and seemingly only half 

 full of fluid, but they soon absorb moisture and become 

 distended. The embryo animal, with its shell, is observ- 

 able in the albuminous fluid in a few days after the egg 

 is laid. Its exclusion takes place, under ordinary cir- 

 cumstances, in from twenty to thirty days, according to 

 the state of the atmosphere. Warmth and humidity 

 hasten the process, while cold and dryness retard it to 

 an almost indefinite extent. The hatching of eggs laid 

 late in the autumn is often interrupted by the approach 

 of cold weather and of snow, and delayed imtil the next 

 spring. 



The young animal gnaws its way out of the egg, and 

 makes its first repast of the shell which it has just left. 

 It consists at first of about one and a half whorls, the 



VOL. 11. 13 



