ZOOLOGY. 



eight testaceous symmetrical plates placed trans- 

 versely. These animals live on rocks and stones 



Haliotis tuberculata. 



on the sea-coast, and are distributed nearly over 

 the whole globe. 



ORDER 2. In the Ophisthobranchiata (Gr. 

 ophisthen, behind, and bragchi, a gill), the shell 

 is rudimentary, and the branchiae are more or 

 less completely exposed on the back and sides 

 towards the posterior extremity of the body. They 

 are commonly called ' Sea-slugs/ and are divided 

 into two sections : 



Section (a). Tectibranchiata comprehends those 

 species in which the gills are protected by the 

 shell or mantle. They are all marine, living 

 chiefly on the shore or on floating sea-weed. In 

 the family of the Bullidce (Bubble-shells), the shell 

 is large and convolute, and in some cases is large 

 enough to receive the greater part of the animal. 

 The Aplysiadce are slug-like in form, and have 

 the mantle very large, and reflected upwards, so 

 as to cover in the gills. They possess a merely 

 rudimentary shell, and owing to their tentacles 

 being turned backwards, like ears, they are popu- 

 larly known as ' Sea-hares.' 



Section (ff). Nudibranchiata, in which the 

 shell is absent, and the 

 branchiae are exposed on 

 some part of the back 

 in the form of a rosette. 

 It includes all the naked 

 marine Gasteropods, as 

 Doris (Sea-lemon), Tri- 

 ton, and Tethys. They 

 are elegant and beautiful little creatures, like 

 slugs, and are generally ornamented with beautiful 

 colours. They are found creeping on sea-weeds 

 or attached to stones at low-water. 



ORDER 3. The Heteropoda have the foot com- 

 pressed into a thin vertically flattened ventral 

 fin, by means of which locomotion is effected. 

 They are free-swimming and pelagic. This order 

 comprises two families, Firolidce and Atlantidce. 

 In the Firolidcs, the shell is absent, or exists in a 

 rudimentary form, and protects the circulatory 

 and respiratory organs, including the genera Cari- 

 naria and Firola. In the Atlantida, such as 

 Atlanta and Bellerophon, the shell is large, and 

 capable of containing the whole animal. 



Section (B). The Pulmonifera are characterised 

 by having their respiratory organs so arranged as 

 to breathe air directly. The mantle is inflected 

 so as to form a pulmonary chamber, into which 

 air is admitted from without, and where it comes 

 in contact with the plenus of vessels with which 

 the wall of the chamber is supplied. They are 

 hermaphrodite. Most of them are land animals : 



Doris : a, gills. 



although a few are aquatic, living chiefly in fresh 

 waters and brackish pools, they are forced to 

 come to the surface to breathe. This section 

 includes two orders. 



ORDER 4. Inoperculata. The animals in this 

 order are not provided with an operculum for 

 closing the shell. The family of the Helicida 

 (Land-snails) are among the most familiarly 

 known of all animals. The shell is large, and 

 capable of containing the entire animal. They 

 feed exclusively on vegetables, and the destruc- 

 tiveness of the Garden-snail (Helix hortensis) is 

 well known. In tropical climates, the genus 

 Bulimus sometimes attains to a great size. The 

 tropical Achatina is distinguished by the beauti- 

 ful colours of its shell. The family of the 

 Limacidce (Slugs) are naked snails, and are 

 very destructive to vegetables. The head bears 



Mature Snail. 



four tentacula, and at the end of the longer pair 

 the eyes are situated. These tentacula (usually 

 called the ' horns ') can be drawn in by a process 

 resembling the inversion of the finger of a glove. 

 The family of the Lymn&tdtz (Pool-snails) reside 

 in stagnant waters, feeding upon plants and seeds. 

 The shell is thin, well developed, with the aper- 

 ture simple, and the lip sharp. The Lymncea 

 is connected with the development of the immature 

 cercaria into distoma. In the genus Planorbis 

 (Marsh-snails), the coils of the shell are all upon 

 the same plane. 



ORDER 5. In the Operculata, the shell is pro- 

 vided with an operculum, which is a horny or 

 calcareous disc attached to the foot, and is drawn 

 into the mouth of the shell by the contraction of 

 the animal It includes the genus Cyclostoma, a 

 snail-like animal provided with a thin spiral shell, 

 with the margins usually reflexed all round. 



CLASS VI. PTEROPODA. 



The Pteropoda (wing-footed) are free-swimming 

 oceanic forms, provided 

 with two wing-like ap- 

 pendages on each side 

 of the anterior extrem- 

 ity of the body. They 

 are small animals, and 

 exist in all seas, but are 

 found in enormous multi- 

 tudes in the Arctic and 

 Antarctic Oceans. One of 

 them, the Clio Borealis, 

 forms the chief food of 

 the whale, which swallows Example of the Pteropoda 

 thousands at a mouthful. (Cleodora fyramidata). 

 They are eminently car- 

 nivorous, feeding upon minute crustaceans and 

 other small animals. 



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