Xll EXPLANATION OF TEEMS USED. 



born differ from their parents only in size ; while in the latter the young pass 

 through a laval state, in which the animal is enclosed in a spiral involute shell, closed 

 by an operculum, and instead of creeping along the bottom they swim in the ocean 

 ^y means of a pair of ciliated fins. In this stage there is hardly any difference 

 between species belonging to widely different families. This embryonic shell may be 

 either entirely lost in the adult, in which case the animal is said to be naked (NUDI- 

 BRANCHIATA), or it may be developed into a univalve or multivalve shell, but never 

 into a bivalve one. The univalve shell may be either CONICAL (Patella), BOAT- 

 SHAPED (Crt/pta), or spiral; in which latter case it may be IRREGULARLY SPIRAL 

 (T'ermei'us), or REGULARLY SPIRAL. The regularly spiral shell may have various 

 shapes, such as TTJRRETED (TurrUella), GLOBULAR (Natica), DEPRESSED (Eotelld), 



CONVOLUTE (Slllla), FUSIFORM (FllSUS), TROCHIFORM (Imperator) , TURBINATED 



(Turbo), or EAR-SHAPED (Haliotis). Nearly all the spiral shells are DEXTRAL or 

 right-handed, but a few (Ceritfiium in part) are SINISTRAL or left-handed. The con- 

 volutions of the shell are called WHORLS, and the AXIS around which the whorls are 

 coiled is sometimes hollow or PERFORATED, and the aperture is called the UMBILICUS. 

 Generally, however, the axis is a solid COLUMELLA (fig. 8a), in which case the shell 

 is said to be iinperforate. The line formed by the junction of two whorls is called 

 the SUTURE (fig. 87*). The last turn of the shell is the BODY WHORL (fig. 81), the 

 others the spire (fig. 8c) ; the BASE is the opposite end to the APEX (fig. 8cl) or 

 point. The APERTURE, or mouth of the shell, is entire in most of the vegetable 

 feeders, but NOTCHED or produced into a CANAL (fig. 8f) at the lower or ANTERIOR 

 end in the carnivorous families. Sometimes there is a short POSTERIOR or upper 

 canal (fig. 8e) near the suture. The outside of the aperture is formed by the outer 

 or RIGHT LIP (fig. 8/7), the inner by the inner or COLUMELLAR LIP. The right lip is 

 often thickened, and when these thickenings are formed periodically they are called 

 VARICES. Lines of sculpture or colour running from the apex to the aperture are 

 termed SPIRAL, while those that 'coincide with the lines of growth are called TRANS- 

 TERSE. The aperture is often closed by a horny or calcareous plate, fixed to the foot 

 of the animal, called the OPERCULUM. 



In most of the Gasteropoda the body is unsymrnetrical, and is coiled up spirally ; 

 it is enclosed in a MANTLE, which is not divided into two lobes. Locomotion is 

 effected by means of a broad muscular disc called the FOOT. The head is generally 

 distinct, and provided with two tentacles and two eyes, which are sometimes placed 

 upon stalks. The mouth contains a long cartilaginous strap called the tongue, which 

 is covered with teeth transversely arranged. 



Lamellibranchiata (fig. 10). 



The Lamellibranchiata have no head, and the body is protected by a bivalve shell, 

 the valves being applied laterally to the animal. They are connected together at the 

 DORSAL MARGIN by an elastic LIGAMENT, and articulate by a HINGE furnished with 

 interlocking TEETH. The shell is closed by one (Monomi/aria) or two (Dimt/aria) 

 powerful ADDUCTOR muscles, and the parts of the shell to which they are attached 

 is always indicated by a scar (ADDUCTOR IMPRESSION), fig. 10c. The border of 

 the mantle is also muscular, and the place of its attachment is marked by a line 

 called the PALLIAL LINE (fig. 10). The presence of a bay or sinus (PALLIAL SINUS), 

 fig. 106, in this line, shows that the animal had retractile siphons. The sinus, when 

 present, and the ligament, are always on the POSTERIOR (fig. 10/") end of the shell. 

 The apex of each valve is called the UMBO (fig. lOrf) or BEAK, and is generally turned 

 towards the ANTERIOR end (fig. lOe). The LENGTH of a bivalve is measured from 

 its posterior to its anterior margin ; the BREADTH from the dorsal to the ventral 

 margin ; and the THICKNESS from the outside of one valve to that of the other. In 

 the Monomyaria the adductor impression is on the posterior side. 



Bivalves are said to be CLOSE when the valves fit accurately, and GAPING when 

 they cannot be completely shut ; EQUIVALTE when the right and left valves are of the 

 same size and shape, and EQUILATERAL when the umbones are placed in the centre. 

 The surface is often ornamented by ribs which RADIATE from the umbones to the 

 margin, or with CONCENTRIC ridges which coincide with the lines of growth. The 

 LUNULE (fig. 10$r) is an oval space in front of the beaks; when a similar impression 

 exists behind the beaks, it is termed the ESCUTCHEON. 



