136 



CLASS MOLL USCA. 



SUB-CLASS ACE P HAL A. 



section (the Asddian) they are connected with the Molluscs. Cuvier, 

 however, observing that they are provided with a brain, heart, vessels, 

 liver, &c., considers them entitled to a higher place in the animal scale, 

 and has ranged them with his ACEPHALOUS MOLLUSCS (a disposition in 

 which Blainville also concurs), bat distinguishing them as Shell-less. They 

 exist either as single independent animals, capable of voluntary motion, as 

 the Salpce ; or are attached to rocks, sea-weed, &c., and are either sessile, 

 i.e., fixed by their broad base like Cynthia (Plate 8, fig. 1), or pedicellate, 

 i>., have a long pedicle, as Clavelina (fig. 2), for their attachment. Others, 

 of the genus Saljta, are remarkable for their capability of aggregating them- 

 selves together in one common cartilaginous mass, like the polypary of 

 AJcyomum. This mass may be either sessile, as Distama (fig. 4), or pedi- 

 cellate, as Sigittina (fig. 13), and in them it is simply indented with cells. 

 But in others, as BotryUus (fig. 8), the central part is hollowed out as a 

 shallow saucer-like cavity, into which the anal apertures of the several 

 animals disposed around it in a circular or oval form empty themselves. 

 The external layer varies materially in its character, and is either almost 

 gelatinous, as in Sigittina, or cartilaginous, as in PhaUusia. The internal 

 layer is either muscular throughout, as in Cynthia microcosmus, or mem- 

 branous, with some muscular bands stretching only over particular regions, 

 as in Phattusia sulcata, and in the different kinds of Salpce, or simply 

 membranous, as in Sigillina Australia. 



2 and 3. The Polliobranchiate and Lamettibranchiate Orders, which include 

 the remaining living Acephals. The name assigned by Blainville to the 

 former Order arises from " the branchiaz being applied to the inner face of 

 the lobes of the mantle ;" while the Lamettibranchiate Order has its name 

 from the gills, branchiae, being disposed upon the body of these animals 

 like the leaves of a book. 



As the animals belonging to these Orders are contained within a pair of 

 shells or valves, as they are called in zoological language, they are said to 

 be bivalves. It will therefore be necessary to give some account of their 

 form and characters. 



The Valves (Conchol. Terms, Plate 1) are placed one on each side of 

 the animal, and united together by an elastic ligament, which, to a greater 

 or less extent, connects the inner edge of the upper or dorsal margin of one 

 valve with that of its fellow, the whole of which apparatus is called the 

 hinge. The anterior end of the valve is- called the oral extremity, from 

 being near the mouth, and the posterior end the anal extremity, from its 

 proximity to the vent of the animal. In most Bivalves the shells shut 

 closely, and no apertures'exist ; but in others, as the Itazorshell, Solen, both 

 ends are open, the valves together having the shape of a truncated, flattened 

 cylinder ; and in others, as Gakomma, the ventral margins do not touch. 

 The form of the valves varies considerably : they may be long, as in the 

 Piddock, Pholas (Conchol. Terms, Plate 1), and Mussel, Mytilus; or deep, 

 as in VulseUa; oval, as in Cytherea (Plate 1) ; rounded, as in the Scallop, 

 Pecten (Plate 1); thick, as in the Cockle, Cardium; compressed and very 

 delicate, as Tellina ; cylindrical, as the liazorshell, Solen; boat-shaped, like 

 the Ark, Area ; heart-sliaped, like the Cockle, Cardium ; wedge-shaped, as 

 the Wedgeshett, Donax ; tongue-shaped, like Vulsella ; beaked, when the 

 hinder extremity of the shell is much narrower than the front one, as in 

 Tellina frayilis ; or fan-shaped, when the hinder end is very broad and as 

 it were truncated, as in the Nacreshett, Pinna; eared, either singly or 

 doubly, when the edge of the shell nearest the beak or summit expands 

 into one ear, as in Unto delphinus, or into two, as in the Scallop. The 

 external surface of the valves is smooth, as in Cytherea Chume ; scaly, as in 

 the Oyster, Ottrea ; radiated, like the Scallop; ribbed, as the Cockle; 

 grooved, as the Aslarte Danmoniensis ; striated, as the Itazorshell ; or tes- 

 sellated, as the Reticulated Ark. According to their correspondence in 

 form, Bivalve shells are said to lie equivalve wlion lx>th ore alike, as in the 

 Mussel; or, when there is but little difference between them, subeqwvalve, 

 as in some of the Scallops ; but if one valve be (lat and the other concave, 

 as in the Oyster, they are called inequivalve. The internal surface of the 

 valve has a generally correspondent concavity with the convex exterior, but 

 it has also some peculiarities of it* own. Wlicn the ventral cavity rises 



into the beak, it is said to be arched, fornicata, as in Isocardia ; when a 

 leaf-like process springs up from its bottom, as in the Arks, it is called 

 chambered, concamerata ; when a lengthened edge descends obliquely from 

 beneath the beak, so as nearly to reach the abdominal edge, as in Anatina, 

 then the valve is said to be solidified, solidijicata ; if, as in the Piddocks, a 

 curved hooking process stretches from the cavity of the beak into the hollow 

 of the valve, it is appendiculate, appendiculata. In the Terebratulce (Plate 1, 

 Lamplike-shells), the right or imperforate valve is furnished with a very 

 remarkable apparatus, consisting of a testaceous loop, commencing near the 

 hinge, stretching into the middle of the shell, and thence turning back upon 

 itself. They are also further remarkable for the aperture near the hinge of 

 the left valve, through which little muscles pass to be connected with the 

 pedicle, which is extended from it like the pedicle of the Barnade. Bivalve 

 shells are closed by the aid of muscular bands which pass from one valve 

 to the other, and their attachments are in pits, more or less deep, in the 

 concavity of the valves ; these pits are called muscular impressions. Some- 

 times there is but a single muscle ; each valve has then only a single im- 

 pression, as in the Oyster, such are called Monomyary valves ; or there may 

 be two muscles, one in front and the other behind, as in Venus, and many 

 others, such are named Dimyary ; in some, as Unio and Anodon, there are 

 three muscles, and the valves therefore Trimyary. 



Another remarkable character of all the Acephalous Molluscs, excepting 

 the Heterobranchiate Order, is their possession of a true mantle, pallium, 

 which is really only an elongation of the common tegument of the animal 

 turned backward loosely upon itself: the Mollusc, therefore, besides its 

 close tegumentary investment, is included between the flaps of the mantle 

 like a book within its fly-leaves, and the connection of the external surface 

 of the mantle-flaps with the interior of the valves corresponds to that of the 

 fly-leaves with the book-covers. Such is the simple description of the 

 mantle ; but it must be recollected that though it envelops the animal as 

 far as its dorsal edge before its reflexion upon the valves, yet that in taking 

 this course it must wrap round the muscle or muscles connecting the 

 valves. This simple double-flapped mantle is easily distinguished in the 

 Oysters, Arks, and Scallops. But the mantle is subject to a variety of 

 form, as any one may satisfy himself by comparing the pallium of the Kazor 

 shell, Cockle, Mussel, and Scallop. 



ORDER HETEROBRANCHIATA. 



ILLUSTRATIVE EXAMPLES. 

 PLATE 8. 



CYNTHIA. A subgenus of Ascidcs according to Savigny, the generic 

 characters of which are : body enveloped in a double tunic ; fixed to marine 

 bodies at the base. Exterior tunic somewhat coriaceous, forming an irregular 

 ovate, or cylindrical sac, perforated above by two unequal foramina, one 

 lower than the other. The interior, or proper tunic enclosing the body, 

 not entirely filling the external sac, to which it is united only at the foramina. 



The species of Cynthia are sessile Ascidians, their branchial sac is plaited 

 longitudinally, and they are found either attached to submarine plants, or 

 floating about in the gulf of Suez. 



Illustrations : Cynthia momus, C. canopus. 



BOTRYLLUS. The common substance gelatinous or cartilaginous, encrust- 

 ing other bodies, and composed of systems which are round or elliptical, 

 raised above the common surface, and annular. Animals disused cither 

 in a single series, or in several which are regular and concentric. Branchial 

 orifice simply circular, and without rays ; intestinal orifice small 

 to a point, and envelojied in the border of the central cavity of the system. 



There are several species found on our coasts, as B. conglomerates, &c. 



Illustration : Botryllus polycyclus. 



POLYCLINUM. Animals aggregated, biforous, sunk in a gelatinous mass, 

 flattened, rough with small papilla: ; the greater part disposed in a radiated 

 form round a centra! opening; mouth willi M\ tenlarula ; anal aperture not 

 apparent externally ; one gemmiferous sac hanging beneath the animal 

 terminating in a filament. 



