320 



Gasteropoda.- 



-MOLLUSCA.- 



-TOXIFERA, 



Sub-Class I. — CTENOBRANCHIATA, Gray- 

 Prosobranchiata, M. Edwards. 



The cavity in which the gills, in this large group, are 

 placed, forms an open bag in the last whirl of the 

 shell, over the back of the neck; and there is a con- 

 stant current of water passing over the gills, entering 

 at the front and making its way out near the inner 



hinder angle of the gill cavity, and mouth of the shell. 

 The sub-class Ctenobranchiata comprehends within 

 itself the greater part of the Gasteropodous mollusca, 

 whicli are furnished with large and well-developed 

 shells. 



Order I.— PECTINIBRANCHIATA. 



This order contains a larger number of species than any 

 other order of Gasteropods, and is divided into three 

 sub-orders, the characters of which are taken in a great 

 measure from the structure and form of the teeth. In 

 the first sub-order, Toxifera, the head is small, and the 

 fleshy proboscis with which it is furnished is retractile 

 within the body. The teeth are elongate, awl-shaped, 

 and implanted in this proboscis. The eyes are sessile, 

 th it is, are not placed on a footstalk, but are situated 

 on the outer side, near or on the end of the feelers 

 or tentacles. In the second sub-order, Probosci- 

 difcra, which has the head small and the eyes sessile, 

 as in the previous section, but the retractile proboscis 

 more lengthened and capable of being extruded to a 



greater extent, the teeth are placed in one or several 

 rows on the surface of a linear, elongate, cartilaginous 

 tongue-membrane. In the third sub-order, Rostrifera, 

 the head is produced into a beak or rostrum, and the 

 teeth are disposed in seven longitudinal rows on an 

 elongated linear tongue-membrane, partly received 

 into the visceral cavity. This disposition of the teeth 

 seems to be nearly connected with the habits of the 

 animals belonging to these sub-orders. The Probosci- 

 difera for instance are carnivorous, or animal feeders ; 

 and the Rostrifera are phytophagous or vegetable 

 feeders ; whilst the Toxifera, which are said to feed on 

 both animal and vegetable matters, inflict rather severe 

 wounds upon those who incautiously handle thera. 



Sub-order I.— TOXIFERA. 



The structure of the teeth brings into this group several 

 genera of shells, some of which have hitherto been 

 placed wide apart from each other. 



Family I.— CONIDvE {The Cones). 



We commence with the Cones, a family rich in 

 species, and the pride of collectors. The animal of 

 the Cones crawls on a long and very narrow foot, 

 which is truncated anteriorly, and bears upon its 

 posterior extremity a small horny annular operculum, 

 with an apical nucleus. Tiiis operculum is very small 

 compared with the size of the shell, is much shorter 

 than the aperture, and so narrow that it allows the 

 animal to retract it deeply into the interior — fig. 50 b. 

 The head is moderately small, and at the extremity of 

 the proboscis, which is a prolongation of the head, is 

 situated the mouth, which is surrounded by a funnel- 

 shaped, rostrum-like veil, slit above.* The teeth, 



• In tlie Cones the foot is small and of weak muscular 

 power, says Dr. Johnston, and we might hence conclude that 

 the animal possesses little facility in locomotion, "yet so rich 

 i^ all creation," he adds, "with proofs of contrivances adapted 

 to annul a defect, that we might anticipate to find some remedy 

 here — and it is so. The mouth of the snail is situated in a 



implanted in the substance of this proboscis, are in two 

 rows, arranged in pairs, and are sharp pointed, more or 

 less swollen at the base, but hollow and barbed at the 

 tip. The mantle of the animal is entire, covers the 

 interior of the shell, and is prolonged anteriorly into a 

 fleshy siphon or canal, cylindrical in form and passing 

 beyond the mouth of the shell, fig. 50 c. This siphon is 

 open at its free extremity, and its use is to carry the 

 water to bathe the gills. As the name imports, the 

 shell is of a conical form, with generally a very short 

 spire ; in many cases the upper whirls being so com- 

 pressed, and the spire so short and flat that the shell 

 can stand erect when placed upon a flat or even surface. 

 The last whirl, indeed, constitutes of itself the greatest 

 portion of its whole surface, and terminates at the 

 base rather by a depression than by a notch. The 

 aperture is narrow, linear — occupying the whole length 

 of the last whirl — and having the lips generally straight 

 and paraUel. The outer lip is simple and sharp-edged, 



cavity, and this cavity it applies to aid its weakness; for it 

 performs, like the oral aperture of the leech, the otBce of a 

 sucker, by which the head is readily affixed to foreign bodies, 

 Adanson ; Senegal^ S9, Thus the animal facilitates its progress, 

 and is enabled to drag along the shell, of a weight and siza 

 otherwise quite burdensome to it." — P. 127, 



