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 Khipidoglossa. 
Sus-OrpER 1. Docoguossa. 
In these Aspidobranchs 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, au), 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. 
Faminy 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. 
Scurria, Gray; with pallial branchiae arranged in a circle beneath the mantle. 
Pectinodonta, Dall. Scenella, Billings ; from the Cambrian. Palaeacmagdy 
Hall; from the Silurian. Faminy 2. TRypirpimpag, Pilsbry. Muscle 
scar divided into numerous separate impressions. Genus—Tryblidiwm, 
Lindstrom ; Silurian. Faminy 3. PATELLIDAE, Guilding. No ctenidia, but 
only pallial branchiae disposed in a circle between the mantle and the 
foot (Fig. 125). 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. Faminy 4. 
Leperipasn, 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. Lepetella, 
Verrill ; with eyes, Pilidiwm, Forbes. Propilidiwm, Forbes and Hanley. 
Faminy 5, BAtHyscIADIDAE, Dautzenberg and Fischer. Monoecious, with 
otoliths ; head provided with an appendage on the right side; radula 
without a central tooth. Genus—bathysciadium, Dautzenberg and 
Fischer ; abyssal (Fig. 126). 
Sup-OrpDrER 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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