3 i8 CHARACTERS OF INVERTEBRATE ANIMALS 



Sub-class i. STREPTONEURA (Prosobranchia) 



This subdivision of Gastropods is partly founded on the 

 course taken by the nerve-loop which is attached to the nerve- 

 ring. It is here twisted, as, e.g., in the Ormer (see p. 310), into 

 a shape resembling the figure 8. Another important feature is 

 afforded by the gill or gills which, when present, are in front of 

 the heart, as again in the Ormer (see p. 308). These Molluscs 

 may therefore be termed " fore-gilled " or prosobranch (Gk. pro, 

 in front of; branchia, gills). They include most of the marine 

 snails which are to be found on the sea-shore. On the shape 

 of the gills, among other characters, the two orders of the sub- 

 class are marked off from one another, i.e. (i) Comb-gilled Snails, 

 with a single gill consisting of an axis bearing a series of small 

 flattened plates, comparable to the teeth of a comb; and (2) Shield- 

 gilled Snails, in which there are two series of such plates, one on 

 each side of the gill-axis. In some members of the second order 

 two gills are present. 



Order (i). Comb-gilled Snails (Ctenobranchia). This order is 

 divided into no less than fifty-nine families, so that space will 

 prevent more than a brief notice of a few common forms. 



Probably no sea-snail is more familiar than the Periwinkle 

 (Littorina littored), common on the rocks between tide-marks, 

 and illustrating a number of points in which the members of 

 the order differ from the Ormer and related forms. The thick 

 rounded shell is obviously spiral, and the visceral hump it covers 

 is of the same shape. The spiral, as in most snails, is a right- 

 handed one, i.e. with its turns running in the same direction as 

 an ordinary screw or corkscrew, so that if the shell be placed on 

 end with apex above, its turns or whorls will be seen to slope 

 up from left to right. The most primitive Molluscs known are 

 bilaterally symmetrical, devoid of a prominent visceral hump, 

 and with a posterior mantle-cavity into which the intestine, &c., 

 open. Such spirally-twisted forms as Periwinkle have apparently 

 arisen from simple forms of the kind by development of a visceral 

 hump, together with a strong shell to cover it, and also to serve 

 as a shelter into which the animal might withdraw itself. At 

 the same time twisting took place, perhaps as a result of the 

 weight of the parts, and the result has been that mantle-cavity, 

 end of intestine, gills, heart, and kidneys have been brought 



