272 DE. H. J. PLEUEE ON THE EVOLUTION OF 



either side, as is the case in the Docoglossa — Pleurotomaria, Haliotis, &c. From the 

 outer one of these two connectives on either side arose tlie nerves of the mantle-resion 

 and the connectives which form the visceral loop. In Haliotis the points of origin are 

 very near that end of the connective which abvits upon the posterior part of the ring, 

 and Tliiele holds this to he the primitive condition. In Pleurotomaria the points of 

 origin are farther forwards, and Woodward believes this type to be primitive, especially 

 as among the Trochidse and Haliotidae there is a tendency for the pleural centre to 

 mount up to the visceral nerve, a specialization along a peculiar line. Among the 

 Docoglossa tlie imperfectly developed pleural centres are not in such close connection 

 with the pedal cords as in Haliotis, but tliey also show the tendency for the pleural 

 centre to mount up the visceral nerve, and perhaps therefore on the whole support 

 Woodward's view, as they are beyond dispute very early Gastropod forms. The pallial 

 nerves were most probably not concentrated into a single trunk ( Woodward). Scissurella 

 resembles the Docoglossa, and therefore Pleurotomaria rather than Haliotis, and so 

 seems to support Woodward's view. The nervous system in general thus seems to 

 have resembled that of Pleurotomaria and the Dtjcoglossa, and in many details, 

 such as the nerve-supply of the slit-region, tliat of Haliotis. The viscei-al loop was 

 certainly smaller than in Haliotis and nearer the median j)lane than in that form or 

 the Docoglossa. 



The Circulatory System (PI. 16. fig. 16 a) no doubt resembled in general features that 

 of tlie Fissurellida?, and to a less extent tliat of Scissurella, Haliotis, and Pleurotomaria. 

 The ventricle was developed around the rectum, and gave rise to an aorta at its posterior 

 end. This aorta sent a blood-channel (visceral artery) to the upper part of the visceral 

 hump, and then went forward to the head to open into a sinus surrounding the cushion 

 of the odontophore. Thence the blood streamed into the lower part of the visceral 

 hump through u sinus surrounding the radular sac, and another connection led the flow 

 of blood into the jjaired longitudinal sinuses surrounding the pedal nerve-cords. The 

 course of the aorta in Haliotis, where, on its way towards the head, it surrounds the 

 radular sac, is certainly a specialization. 



The Respiratory System included a pair of subequal ctenidia in a median or nearly 

 median anterior branchial cavity, Avhose roof contained a pair of mucous glands. In 

 Pleurotowaria and Haliotis we find a secondary elongation of the ctenidia and hyper- 

 trophy of the mucous glands. The right ctenidium and corresponding mucous gland 

 and osjjhradium were smaller than the corresponding organs of the other side. 



There were certainly two kidneys, right and left of the rectum and pericardium. 

 The right kidney was functionally by far the more important of the two and possessed 

 several intervisceral lobes, showing a structure resembling that in Haliotis and Trochus. 

 A right reno-pericardial pore has been found in practically all the archaic Gastropods, 

 and therefore must have been present in the Prostrej)toneure. 



The condition of the left kidney is not so easy to ascertain. This organ is very small 

 among the Docoglossa, but possesses some amount of excretory tissue and retains the 

 reno-pericardial canal (PL 17. fig. 24). Among the Pissurellidae it is reduced almost to 

 vanishing-point. In Pleurotomaria, Haliotis, and the Trochidae, on the other hand, it is 



