LIMPETS AND KEYHOLE LIMPETS. 631 



prolonged in front of this cavity into a respiratory siphon : a posterior exha- 

 lent siphon is likewise present in certain forms. 



Originally, in all probability, the animal, like the Amphineura, had a 

 straight alimentary canal, terminating behind with a gill on either side of the 

 opening ; but, through the operation of causes which need not here be dis- 

 cussed, the alimentary canal first became arched, then its hinder portion was 

 tucked- under, so to speak, whilst subsequently the visceral mass became 

 twisted, the hinder portion, with the gills, being forced round by the right 

 side to the front, so that in its final stage what had been the left gill is, in 

 the typical gastropod of to-day, now on the right side, whilst the right one 

 has passed over the neck to the left side. This last, however, is wanting in 

 the majority of Gastropoda. 



In the process of this torsion the visceral nerve-loop became twisted into a 

 figure of 8, and, though in one great group the body is again untwisted, 

 traces of its former contortion can still be traced in the nerve-loop. 



The Gastropoda are divided first into two sub-classes : A, Streptoneura, in 

 which the visceral nerve-loop forms a figure of 8 ; and B, Euthyneura, in which 

 the loop is not thus twisted. 



SUB-CLASS A. STREPTONEURA. 



To this section the name Prosobranchiata has also been applied, since the 

 gills are in advance of the heart. There is only a single pair of tentacles, 

 the eyes being situated on prominences near their bases. The sexes are 

 usually distinct. 



They are further sub-divided, according to the structure of their gills, into 

 Scutibranchiata and Pectinibranchiata. 



ORDER I. SCUTIBRANCHIATA 



(Sometimes called Diotocardia because the heart has two auricles) comprises 

 the Limpets, Keyhole Limpets, Ormers, and Top-Shells. 



For convenience they are further sub-divided, according to the arrangement 

 of the teeth on the radula, into Docoglossa and Rhipidoglossa. 



Sub-order 1. The DOCOGLOSSA, or limpets, are characterised by their simple 

 conical shell without any operculum. The animals adhere by the sucker- 

 like foot to rocks between tide-marks. There are three families : The 

 ACM^:ID^E in which there is but one true gill. The genus Scurria has in 

 addition a series of leaflets arising from the mantle in a circle between it and 

 the foot, that also serve for the purpose of respiration, and are known as 

 pallial gills. The PATELLID.E, to which the common limpet belongs, have a 

 circlet of pallial gills, but nearly all trace of the ordinary gill has disappeared. 

 The LEPETID^E, going a step further, have neither ordinary nor pallial gills, 

 nor have they any eyes. 



Sub-order 2. RHIPIDOGLOSSA. The radula of this group is very distinctive. 

 The teeth are numerous and weak, the large number of uncini being 

 especially remarkable. 



The FISSURELLID.E, called keyhole limpets on account of the aperture in 

 the shell that communicates with the pallial chamber, where the two gills 

 are. In Emargimda, the "keyhole" takes the form of a notch in the edge 

 of the shell. 



