156 BREATHING ORGANS IN PROSOBRANCHIATA chap. 



becomes several inches long, and is set with formidable spines 

 (see Fig. 164, p. 256). In Bolium 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 vegetarian 

 tendencies of most marine Prosobranchiata, which have been, on 

 this basis, subdivided into Slphonostomata and Holostomata. But 

 this classification is of no particular value, and is seriously 

 weakened by the fact that Natica, which is markedly ' holo- 

 stomatous,' is very carnivorous, while Ceritliium, which has a 

 distinct siphonal notch, is of vegetarian tendencies. 



In the Zygobranchiata the water, after having aerated the 

 Ijlood in the branchiae, usually escapes by a special hole or lioles 

 in the shell, situated either at the apex {Fissurella) or along the 

 side of the last whorl {Haliotis). In Fleurotomaria 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 Ijefore it reaches the liranchiae (see Fig. 

 179, p. 266). 



In Patella the l^reathing arrangements are very remarkable. 

 In spite of their apparent external similarity, this genus possesses 

 no such symmetrically paired plume-shaped branchiae as Fis- 

 surella, 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 morpho- 

 logically related to the paired brancliiae in other MoUusca, 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 tlie right and left sides of the back of the ' neck.' A 

 precisely similar abortion of the true l)ranchiae, and special 

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

 in Phyllidia and Pleurophyllidia (see below under Opistlio- 

 branchiata). This circlet of functional gills in Patella has there- 

 fore little systematic value, being only developed in an unusual 

 position, like the eyes on the mantle in certain Pelecypoda , to supply 

 the place of the true organs which have fallen into disuse. Accord- 

 ingly Cuvier's class of CydohrancJiiata, which included Patella 

 and Chiton, has no value, and has indeed long been discarded. 



