THE GASTROPODA 145 



the right kidney. In the Neritacea, however, there is only one 

 kidney, namely, the left kidney, with a slit-shaped external aperture, 

 and there is a distinct genital orifice, the oviduct being diaulic in 

 the Neritidae. 



The order Aspidobranchia includes the most archaic Gastropods : 

 it includes two sub-orders, the Docoglossa and Rhipidoglossa. 



SUB-ORDER 1. DOCOGLOSSA. 



In these Aspidobranclis the nervous system (Fig. 93) is without dialy- 

 neury, that is to say, there is no anastomosis between the pleural ganglia 

 and the visceral nerve of the opposite side through the intermediary of 

 the pallial nerve. The eyes are open and devoid of a crystalline lens. 

 There are two osphradia, but neither hypobranchial glands nor operculum. 

 The mandible is unpaired and dorsal. The radula generally has trabeculi- 

 form teeth (Fig. 74, F), and there are at most three marginal teeth on 

 either side. The heart has only a single auricle (Fig. 82, cm), and neither 

 it nor the pericardium are traversed by the rectum. The visceral mass is 

 cone-shaped, without a spire. The sub -order includes about 1400 

 species. 



FAMILY 1. ACMAEIDAE, Philippi. A single bipectinate ctenidium, 

 free for the greater part of its extent, is present on the left side (Fig. 82). 

 Genera Acmaea, Eschsholtz ; without pallial branchiae ; Great Britain. 

 Xcurria, Gray; with pallial branchiae arranged in a circle beneath the mantle. 

 Pectinodonta, Dall. Scenella, Billings ; from the Cambrian. Palaeacma, 

 Hall ; from the Silurian. FAMILY 2. TRYBLIDIIDAE, Pilsbry. Muscle 

 scar divided into numerous separate impressions. Genus Tryllidium, 

 Lindstrom ; Silurian. FAMILY 3. PATELLIDAE, Guilding. No ctenidia, but 

 only pallial branchiae disposed in a circle between the mantle and the 

 foot (Fig. 1 25). Genera Patella, Linnaeus ; the pallial branchiae forming 

 a complete circle ; no epipodial tubercles ; British seas. Ancistromesus, 

 Dall ; radula with an unpaired central tooth, which is wanting in Patella. 

 Nacella, Schumacher ; branchial circle complete ; epipodial tentacles 

 present. Helcion, Montfort ; circlet of pallial branchiae interrupted 

 anteriorly, beneath the head ; British seas. Helcioniscus, Dall. FAMILY 4. 

 LEPETIDAE, Gray. Dioecious, with otoconia ; the head symmetrical, the 

 foot elongated ; neither ctenidia nor pallial branchiae present ; a central 

 tooth in the radula. Genera Lepeta, Gray ; without eyes. Lepdella, 

 Verrill ; with eyes. Pilidium, Forbes. Propilidium, Forbes and Hanley. 

 FAMILY 5. BATHYSCIADIDAE, Dautzenberg and Fischer. Monoecious, with 

 otoliths ; head provided with an appendage on the right side ; radula 

 without a central tooth. Genus tiathysciadium, Dautzenberg and 

 Fischer; abyssal (Fig. 126). 



SUB-ORDER 2. RHIPIDOGLOSSA. 



Aspidobranchia with a dialyneurous nerve-system, that is to say, with 

 a pallio-visceral anastomosis (Fig. 124, XV); eyes with a crystalline lens 

 (Fig. 100, I) ; a single osphradium, except in genera with two ctenidia ; 

 one or two hypobranchial glands. Mandibles paired, lateral. Radula 



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