Class 2. Gastropoda. 



301 



the Prosobrancliiata and Pulmonata are viviparous, the eggs developing witMn 

 the oviduct. 



The eggs of Gastropods are always small, aud undergo total 

 segmentation. In the Prosobrauchs and Opisthobranchs the larva 

 passes through a metamorphosis; vs^hen it leaves the egg it 

 usually possesses a well-developed velum, by means of which it moves 

 about in the water : the foot on the contrary is only feebly developed. 

 The young Pulmonates have no velum, and 

 undergo no such transformation. 



Amongst those Prosobranohs which lay egg capsules 

 containing many eggs, a few, or only one in each capsule 

 develops, the undeveloped eggs ai'e swallowed hy the 

 young animal as it swims about in the albumen. Fre- 

 quently the young of such forms undergo metamorphosis 

 within the egg-capsule, which they leave when the velum 

 has disappeared, the foot is formed and the body has 

 reached a considerable size (Whelk, etc.). 



The Gastropods are, for the most part, creep- 

 ing animals, gliding over surfaces by means of 

 undulatory contractions of the foot; not a few 

 {e.g., freshwater forms) have the power of at- 

 taching themselves, so to speak, to the surface 



of the water with the foot upwards, the visceral mass hanging below, 

 and, in this position, they are able to move slowly along. Some 

 smaller divisions of marine Gastropods are distinguished by the fact 

 that they can effect actual swimming movements by means of the 

 modified foot, or by some special organ. The majority of the 

 Gastropods (the Opisthobranchs, and by far the greatest number 

 of the Prosobranohs) live in the sea, not a few in fresh water 

 (some Pulmonata and some of the Prosobranchiata), many are 

 terrestrial (the majority of the Pulmonata and some of the 

 Prosobranchiata) . 



The following table gives a summary of the chief characters of the 

 Gastropoda: 



rig. 247. Larva of 

 a Gastropod (Opistho- 

 branoh). / foot, h au- 

 ditory organ, op opercu- 

 culiun, s shell, v velum. 



Prosobranchiata. 



Sexes separate. 

 Visceral loop in the form 



of an 8. 

 The auricle anterior to 



the ventricle. 

 Respiration (usually) by 



a gUl. 

 Penis projecting freely. 



Metamorphosis. 



Opisthobranchiata. 



Hermaphrodite. 

 Visceral loop, (J -shaped. 



The aiuicle usually poste- 

 rior to the ventricle. 

 Respiration by giUs. 



Penis capable of invagina- 

 tion. 

 Metamoi-phosis, 



Pulmonata. 



Hermapbi-odite. 

 Visceral loop, (J -shaped. 



The atu-icle anterior to 

 the ventricle. 



Respii-ation by a pul- 

 monary chamber. 



Penis capable of invagi- 

 nation. 



No Metamorphosis. 



