GENERAL NOTES. 5 



has been estimated that one of the spawn-clusters of the Common 

 Squid (Loligo vulgaris) contains as many as 40,000 ova. Every- 

 body knows the spawn-cases of the Common Whelk, found so abun- 

 dantly on the sea-beach, consisting of a large number of yellowish 

 capsules, heaped one upon another and forming an irregularly 

 rounded mass. As many as five or six hundred capsules may be 

 piled together in a single heap, each capsule containing several 

 hundred eggs, of which, however, perhaps only thirty or forty are 

 hatched. 



In other families, as Aplysia, Doris, Eolis, &c, the eggs are 

 contained in a spirally rolled ribbon or strap-like structure; and 

 some of the Natica: build a somewhat similar capsule, composed of 

 the eggs cemented together by sand and a gelatinous material, the 

 whole forming two thirds of a circle narrowed at the upper part. 



Terrestrial Mollusks deposit, in comparison with their marine 

 relations, but very few eggs. They are sometimes covered by a 

 thin soft skin, but in certain groups, such as the large South- 

 American Bulimi and the African Achatince, which include the 

 largest of known land-mollusks, they are protected by a hardened 

 calcareous shell, in some instances fully an inch in diameter. The 

 freshwater forms (Limncea and Physa) deposit from thirty to a 

 hundred eggs enveloped in a gelatinous mass. 



Ihe number of eggs produced by some Bivalves is enormous. 

 The Common Oyster is said to produce a million or more, and the 

 American variety ten, or even sixty, times as many. Some of the 

 River-Mussels are also very prolific, as many as two millions being 

 sometimes the product of a single individual. A small series of the 

 eggs of Land-Snails and of the egg-capsules of some marine Gastro- 

 pods is exhibited in Table-case G near the entrance to the Gallery. 



The ova of Mollusca may be gradually developed into the form 

 of the parent, or there may be a free-swimming larva, which has 

 a circlet of cilia near the anterior pole of its body (so-called 

 " Veliger" larvae), or there may be special larvae, as in the case of 

 the Freshwater Mussel, the " Glochidmm " as it is called, which 

 has a toothed bivalve shell by which it can fix itself to fishes. 



The limits of age of mollusks has been definitely ascertained in Duration 

 a few instances only. Most Land-Snails probably live about two of life, 

 years, although in confinement some have been kept alive for a 



