GASTROPODA. 757 



sition between the Branchiate and Pulmonate groups is effected by 

 forms like Ampullaria, in which gills are present, but the walls of 

 the mantle-cavity are in parts highly vascular, and are thus adapted 

 for aerial respiration. Another link between these two groups is 

 afforded by forms like the Cyclostomida and the Helicinidce, in 

 which the general organisation of the animal is that of the Proso- 

 branchiate Gastropods, but the breathing-organ is a pulmonary 

 chamber. 



A few Gastropods retain the eggs within the uterus until they are 

 hatched ; but the majority are oviparous. The eggs are often laid 

 in the form of a string or band (" nidamental ribbon ") ; or they may 

 be enclosed in horny capsules (as, for example, in the common 

 Whelk). The young, when first hatched, are provided with an 

 embryonic shell, which in the adult may become concealed in a 

 fold in the mantle, or may be entirely lost. In the common spiral 

 Univalves the embryonic shell remains at the summit of the spire, 

 as the " nucleus " of the adult shell. In the branchiate Gastropods 

 the embryo is protected by a small nautiloid shell, and passes 

 through a "veliger" stage, swimming freely by means of a ciliated, 

 often lobed "velum." Among the Pulmonate Gastropods, those 

 which are strictly terrestrial pass through no metamorphosis, the 

 " velum " being absent in the embryo. 



As regards their classification, the Gastropods may be divided 

 into the two primary groups, or sub-classes, of the Branchiogastropoda 

 and the Puhnogastropoda, the general distinction between the two 

 divisions being that the animal in the former is adapted for an 

 aquatic mode of respiration, while in the latter the breathing-organ 

 is a pulmonary sac. The division of the Branchiate Gastropods 

 may, again, be divided into the four orders of the Prosobranchiata 

 (the ordinary Univalves), the Opisthobranchiata (the Sea-slugs), the 

 Pteropoda (the Winged Snails), and the Heteropoda ; while the Pul- 

 monate Gastropods may be divided into the two orders of the 

 Stylo7n?natophora and Basomniatophora, in accordance with the 

 position of the eyes. The Chitons (Polyplacophora) and the 

 Dentaliidce (Scap/zopoda), often included among the Gastropoda, 

 will be here regarded as constituting two separate classes of 

 Mollusca. 



As regards their distribution in space, the Heteropods, the Ptero- 

 pods, the Opisthobranchiates, and the great majority of the Proso- 

 branchiates are inhabitants of the sea. Certain groups of the Proso- 

 branchiates, however, are found either in brackish or in fresh waters ; 

 while the terrestrial groups of the Cyclostomidaz, Aciculidce, and Heli- 

 ci?tidce, often placed among the Pulmonate Gastropods, are now 

 usually regarded as being essentially Prosobranchiates modified for 

 an aerial mode of respiration. The Pteropods and the Heteropods 



