3332 THE MOLLUSKS OR SHELLFISH 



islands at a depth of eight hundred fathoms, and 5. gracilis off the Azores in one 

 thousand fathoms. By far the largest known form is the common British S. 

 lignarius, which ranges from Finmark to the Mediterranean. 



The bubble shells (Bullidcz}, as this group is popularly called, are something 

 like Scaphander in form, but rather rounder and stronger in their structure. 

 Most of those belonging to the typical genus Bulla are prettily painted with 

 blotches, clouds, and dots. The animal is not too large for its shell, has distinct 

 eyes, and a different type of radula to that of Scaphander, which these mollusks 

 resemble in their predatory habits. Another genus is represented in Britain by 

 Acera bullata, which occurs on oozy ground and mud flats in many estuaries. It 

 also ranges from Finmark to the Mediterranean; and is remarkable for its thin, horny, 

 convolute shell, with a slit at the suture, through which the animal protrudes a 

 long, thread-like mantle appendage. The side lobes are largely developed, and 

 can be reflexed so as to completely envelop the shell. They are also employed in 

 swimming. Another family is represented by Ringuula, all the members of 

 which are very small, not a quarter of an inch in length. The shells are entirely 

 white, more or less globular, with a pointed spire, and with the aperture which is 

 notched in front to some extent contracted with folds. The channeled character- 

 istic of the aperture is rather exceptional, the shells of the Tectibranchs being almost 

 exclusively holostomatous, that is, with uninterrupted apertures. Gastropteron, 

 which typifies another family, is remarkable for the great lateral expansions, or 

 epipodia, which are used by the animal in swimming. The gill lies exposed on 

 the right side, the head shield is truncate in front and pointed behind, and the 

 mantle terminates behind in a slender, whip-like appendage. At one time this 

 mollusk was supposed to be shell -less, but in reality an internal shell does exist; 

 but, as it is only one-fiftieth of an inch in diameter, it may be easily overlooked. G. 

 meckeli of the Mediterranean is of a vivid red color, with 

 a few white spots, and a pale or bluish border to the lobes. 

 In the Philinidce, as typified by Philine, the shell is very 

 thin, and wholly concealed beneath the mantle. In form 

 it somewhat resembles Bulla, but has ithe aperture larger. 

 P. aperta, a very common British species, lives in shallow 

 water, all round the coast, feeding upon foraminifera and 

 Philine aperta. even small sea urchins. The animal is white, has a very 

 (Natural size.) large frontal shield, but no tentacles or eyes. It is provided, 

 as in the case of Scaphander, with a powerful gizzard. The 



ova, which are very numerous, are deposited in a single series, in a long and loosely- 

 twisted cord, inclosed within an ovoid, gelatinous capsule. 



The sea hares (Aplysiidtz] , so called on account of a slight resemblance to a 

 crouching hare, and not for their nimbleness of foot, are elongate creatures, with 

 a long neck, distinct head, oral and dorsal tentacles, and furnished beneath the 

 mantle with a shelly plate to protect the branchiae. The mouth is provided with 

 horny jaws, and the gizzard is armed with horny spines, which prepare the food for 

 digestion. The sides have large thin lobes, which are either folded over the back, 

 or used in swimming. Aplysia includes both animal and vegetable feeders, inhabit- 



