Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/676

Rh 048 MOLLUSCA [AZYGOBEANCHIA. between the mass of the liver and the muscular foot. The radula has 160 rows of teeth with twelve teeth in each row. Two pairn of salivary ducts, each leading from a salivary gland, open into the buccal chamber. The oesophagus leads into a remarkable stomach, plaited like the manyplies of a sheep, and after this the intestine takes a very large num ber of turns embedded in the yellow liver, until at last it passes between the two renal sacs to the anal papilla. A curious ridge (spiral ? valve) which secretes a slimy cord is found upon the inner wall of the intestine. The general structure of the Molluscan intestine has not been suffi ciently investigated to render any comparison of this struc ture of Patella with that of other Mollusca possible. The eyes of the Limpet (28) deserve mention as examples of the most primitive kind of eye in the Molluscan series. They are found one on each cephalic tentacle, and are simply minute open pits or depressions of the epidermis, the epidermic cells lining them being pigmentedand connected with nerves (compare fig. 118). The Limpet breeds upon the southern Eng lish coast in the early part of April, but its de velopment has not been followed. It has simply been traced as far as the formation of a Diblastula which acquires a ciliated band, and becomes a nearly spherical Trocho- sphere. It is probable that the Limpet takes several years to attain full growth, and during that period it frequents the same spot, which becomes gradually sunk below the surrounding Fio. 3C ._ Nervous system of Fissurella. J)7, Surface, especially if the pallial nerve; j&amp;gt;, pedal nerve; A, abdomi- vnrk IIP pnrhrmfltpnf limr&amp;gt; nal 8 an S lia in tlle Streptoneurons visceral - Cai Donate 01 lime. commissu re, with supra- and sub-intestine At low tide the Limpet ganglion on each side ; B, buccal ganglia ; /i . j. j.i jT &i k &amp;gt; cerebral ganglia ; es, cerebral commis- ( being a Strictly inter- sure ; , otocysts attached to the cerebro- tidal Organism&quot;) is ex pedal connectives. (From Gegenbaur, after j A xi. j Jeering.) posed to the air, and is to be found upon its spot of fixation ; but when the water again covers it, it (according to trustworthy observers) quits its attachment and walks away in search of food (minute encrusting algse), and then once more as the tide falls returns to the identical spot, not an inch in diameter, which belongs, as it were, to it. Several million Limpets twelve million in Berwickshire alone are annually used on the east coast of Britain as bait. Order 2. Azygobranchia. Characters. Streptoneura which, as a sequel to the torsion of the visceral hump, have lost by atrophy the originally left ctenidium and the originally left nephridium, retaining the right ctenidium as a comb-like gill-plume to the actual left of the rectum, and the right nephridium (that which is the smaller in the Zygobranchia) also to the actual left of the rectum, between it and the gill-plume. The right olfactory organ only is retained, and may assume the form of a comb-like ridge to the actual left of the ctenidium or branchial plume. It has been erroneously described as the second gill, and is known as the para- branchia. The rectum itself lies on the animal s ri^ht shoulder. The presence of glandular plication of the surface of the mantle-flap (fig. 46, x) and an adrectal gland (purple- gland, fig. 47, yp) are frequently observed. The sexes are always distinct; a special genital duct (oviduct or sperm duct) unpaired is present, opening either by the side of the anus or, in the males, on the right side of the neck in con nexion with a large penis. The shell is usually large and spiral; often an operculum is developed on the upper sur face of the hinder part of the foot. The dentition of the lingual ribbon is very varied. In most cases the visceral hump and the foot increase along axes at right angles to one another, so that the foot is extended far behind the visceral hump in the ab-oral direction, whilst the visceral hump is lofty and spirally twisted. This is a very large group, and is conveniently divided into two sections, the Reptantia and the Natantia. The former, containing the immense majority of the group, breaks up into three sub-orders, the Holochlamyda, Pneu- monochlamyda, and Siphonochlamyda, characterized by the presence or absence of a trough-like prolongation of the margin of the mantle-flap, which conducts water to the respiratory chamber (sub-pallial space where the gill, anus, &c., are placed), and notches the mouth of the shell by- its presence, or again by adaptation to aerial respira tion. The sub-orders are divided into groups according to the characters of the lingual dentition. In some Azygo brancliia the mouth is placed at the end of a more or less elongated snout or rostrum which is not capable of intro version (Rostrifera) ; in the others (Proboscidifera) the rostrum is partly invaginated and is often of great length. It is only everted when the animal is feeding, and is with drawn (introverted) by the action of special muscles ; the over-worked term &quot; proboscis &quot; is applied to the retractile form of snout. The term &quot; introversible snout,&quot; or simply &quot;introvert,&quot; would be preferable. The presence or absence of this arrangement does not seem to furnish so natural a division of the Reptant Azygobranchia as that afforded by the characters of the mantle-skirt. Section a. REPTANTIA. Characters. Azygobranchia adapted to a creeping life ; foot either wholly or only the mesopodium in the form of a creeping disc. Sub-order 1. Holochlamyda. Characters. Reptant Azygobranchia with a simple margin to the mantle-skirt, and, accordingly, the lip of the shell unnotched ; mostly Rostrifera (i.e., with a non-introversible snout), and vege tarian ; marine, brackish, fresh-water, terrestrial. a. Rhipidoglossa (x.4 to 7.1.4 to 7.x). Family 1. Trochidie. Genera : Turbo, Lin. ; Phasianella, Lam. ; Imperator, Montf. ; Trochus, Lin. ; Rotclla, Lam. ; Euomphalus, Low. Family 2. Ncritidee. Genera : Nerita, L. ; Ncritina, Lam. ; Pilcolus, Low ; Navicetta, Lam. Family 3. Plcurotomarwtae. Genera : Pleurotomaria, Defr. ; Anatomus, Montf. ; Stomatia, Helbing. /3. Ptenoglossa (x.O.x). Family 4. Scalaridee. Genus : Scalaria, Lam. Family 5. Janthinidse. Genera : Janthina, Lam. (fig. 44) ; Rccluzia, Petit. 7. T&nioglossa (3.1.3). Family 6. Ccrithidae. Genera : Ccrithium, Brug. ; Potamides, Brong. ; Nerintea, Defr. Family 7. Melanidie. Genera : Melanin, Lam. ; Mclanopsis, Fer. ; Ancylotus, Lay. Family 8. Pyramidellidie. Genera : Pyramidclla, Lam. ; Stylina, Flem. ; Aclis, Loven. Family 9. TurritcUidee. Genera : Turritella, Lam. ; Caecum, Flem. ; Vermetus. Adans. ; Siliquaria, Brug. Family 10. Xenopliovidse. Genus : Phorus, Montf. (fig. 39).