156 BREATHING ORGANS IN PROSOBRANCHIATA chap. 



many of the Muricidae (e.g. M, haustellum, tenuispina^ trihulus) 

 the canal becomes several inches long, and is set with formi- 

 dable spines (see Fig. 164, p. 256). In Dolium and Cassis the 

 canal is very short, but the siphon is very long, and is reflected 

 back over the shell. 



The presence or absence of this siphonal notch or canal 

 forms a fairly accurate indication of the carnivorous or vege- 

 tarian tendencies of most marine Prosobranchiata, which have 

 been, on this basis, subdivided into Sipkonostomata and Holosto- 

 mata. But this classification is of no particular value, and is 

 seriously weakened by the fact that Natica^ which is markedly 

 ' holostomatous,' is very carnivorous, while CeritJiium^ which has 

 a distinct siphonal notch, is of vegetarian tendencies. 



In the Zygobranchiata the water, after having aerated the 

 blood in the branchiae, usually escapes by a special hole or holes 

 in the shell, situated either at the apex QFissurella) or along 

 the side of the last whorl QHaliotis'). In Pleurotomaria the slit 

 answers a similar purpose, serving as a sluice for the ejection of 

 the spent water, and thus preventing the inward current from 

 becoming polluted before it reaches the branchiae (see Fig. 

 179, p. 266). 



In Patella the breathing arrangements are very remarkable. 

 In spite of their apparent external similarity, this genus pos- 

 sesses no such symmetrically paired plume-shaped branchiae as 

 Fissurella^ but we notice a circlet of gill-lamellae, which extends 

 completely round the edge of the mantle. It has been shown 

 by various authorities that these lamellae are in no sense mor- 

 phologically related to the paired branchiae in other Mollusca, 

 but only correspond to them functionally. The typical paired 

 branchiae, as has been shown by Spengel, exist in Patella in 

 a most rudimentary form, being reduced to a pair of minute 

 yellow bodies on the right and left sides of the back of the 

 ' neck.' A precisely similar abortion of the true brand liae, and 

 special development of a new organ to perform their work, is 

 shown in PhylUdia and PleurophylUdia (see below under Opis- 

 thobranchiata). This circlet of functional gills in Patella has 

 therefore little systematic value, being only developed in an 

 unusual position, like the eyes on the mantle in certain Pelecy- 

 foda^ to supply the place of the true organs which have fallen 

 into disuse. Accordingly Cuvier's class of Cyclohranchiata^ 



