ACEPHALES. 369 



side of it, and the combs are alike : the conical tentacula have their eyes at their external base : tht 

 sides of the foot are garnished with a row of filaments. 



Emarffiiiula, Lam., has exactly the same structure as Fissurella; but instead of a hole in the apex, its cloak and 

 shell have a little cleft or emargination on tlieir anterior side, which also penetrates into the branchial cavity. The 

 margins of the cloak envelope and in a great measure cover those of the shell : the eyes are on a tubercle at the 

 outer bases of the conical tentacula ; and the sides of the foot are as usual ornamented with filaments. 



Parmophorus, Lam. (Sciitum,^loxiti.)—\s in Emarginula, the shell is covered, in a great measure, by the turned- 

 up margins of the cloak : the branclii;e and other organs are the same as in the two preceding genera ; but the 

 oblong, slightly conical shell has neither hole nor emargination. [Sowerby unites this with the preceding genus.] 



THE NINTH ORDER OF THE GASTEROPODES. 



THE CYCLOBRANCHIATA.* 



These Mollusks have their branchiae in the form of little leaflets or pyramids, attached in a 

 circle, more or less complete, under the margins of the cloak, very nearly as in the Inferobran- 

 chiata, from which they are distinguished by the nature of their hermaphroditism ; for, as in 

 the preceding order, they have no organs for copulation, and impregnate themselves. Their 

 heart does not embrace the rectum, but varies in its position. We know only two genera, whose 

 shell never exhibits even a trace of a spire. 



The Limpets {Patella, Linn.) — - 

 Have the body entirely covered with a conical shell ; and under the margins of their cloak there is a 

 circle of branchial leaflets. The anus and the orifice of the organs of generation are a little to the right 

 above the head, to which there is a thick, short proboscis, and two setaceous tentacula, having the eyes 

 at their exterior bases : the mouth is fleshy, and contains a [very long ribbon-like] spinous tongue, 

 which is directed backwards, and lies folded deep within the interior of the body. The stomach is 

 membranous, and the intestine long, slender, and much convoluted. The heart is in front above the 

 neck, a little to the left. Some species occur in abundance on our shores. 



The Chitons {Chiton, Linn.) — 

 Have a series of testaceous symmetrical plates set along the back of their cloak, but not occupying 

 all its breadth. The margins of the cloak itself are coriaceous, either naked, or chagreened, or gar- 

 nished with spines, or hairs, or bundles of bristles. Beneath this margin, on each side, is a row of 

 lamellated branchia; ; and in front, a membranous veil over the mouth holds the place of tentacula. The 

 anus is under the posterior extremity. The heart is situated behind, upon the rectum. The stomach 

 is membranous, with a long convoluted intestine. The ovary lies above the other viscera, and appears 

 to open upon the sides by two oviducts. 



There are some small speceies on our shores ; but in the seas of tropical countries they attain a much greater 

 size. (The C7«7o«eW!<*, Lam., distinguished by the valves being so small as only partially to cover the cloak, 

 should be re-united to Chiton, which, in the system of Blainville, forms a separate class, named Pohjplaxiphora, 

 and which, he supposes, leads the way to the Articulated Animals.) 



THE FOURTH CLASS OF MOLLUSCA. 



THE ACEPHALES.f 



The Acephales have no apparent head, but a mouth only, concealed in the bottom, 

 or between the folds, of their cloak. The latter is almost always doubled in two, and 

 incloses the body as a book is inclosed between its covers ; but it frequently happens 



* In the system of Blainville the Cyclobrandiiata is an order that 

 embraces the Uoris. With the last three genera of the preceding 

 order, and with the Patella!, he makes his oMer Cervico-brnnchiutti, 

 divl.led into the Ketiferes and Brauchijeret : the Retiferes are the 

 Prvtfll.-c ; for lie supposes that they breathe by moans of a vascular 

 network in the ciivilv ;^ituatcd above the iiead. 1 have not been able 



to discover it, nor indeed to see any other organ of respiration except 

 that of a cord of leaflets which encircles the body under the margins 

 of the cloak. 



r M. de Blainville unites my Acephales and Branchiopodes in one 

 class, his Acrphalophora. 



B U 



