358 



Pteropoda.- 



-MOLLUSCA.- 



-CONCIIIFERA. 



Genus Limacina. — This is the ouly genus in the 

 family; the shell is subglobose, with a slightly raised 

 spire, the last whirl obscurely keeled, and the axis 

 umbilicated. 



Ik AB.CTICA is the best known species, and is 

 extremely abundant in the North Sea, where it forms 

 an important article of food for the whale. It was well 

 known to some of our early arctic voyagers, and is 



often mentioned by them. Peyrere, in his " Histoire 

 du Greenland, " calls it, " a little black spider." Martens, 

 in his "Voyage into Spitzbergen and Greenland," calls 

 it the " snail slime fish," and tells us that they " swim 

 in great numbers in the sea, as numerous as the dust in 

 the sun," whilst Otho Fivbricius, in his "Fauna of 

 Greenland," gives us a full and very interesting descrii> 

 tion of it. 



Order II.— GYMNOSOMATA (Siiell-less Pteropods). 



These naked-bodied or Shell-less Pteropods are distin- 

 guished fp'jm the preceding order by their body being 

 destitute of a shell. It is composed of three families, 

 i.one of them containing more than a few species, and 

 very few of these being mucli known. No doubt many 

 more species will be discovered floating on the seas of 

 warm latitudes ; but at present the most interesting are 

 one or two found in our Northern Ocean. 



Family— CLIONID^. 



The Clioa have a fusiform body, furnished with two 

 fins, and a central foot-like appendage. Their head is 

 furnished with a series of conical prominences on each 

 side, which probably represent rudimentary tentacles; 

 they are retractile, and furnished with numerous micro- 

 scopic suckers. 



The Genus Clio (=CT!bn«)is the bestknowuof the 

 fimily, as one of the species belonging to it is a native 

 of our arc'.io seas, and has been frequently observed in 

 its native haunts i)y our arctic voyagers. 



THE CLIO BOEEAIIS is a small, gelatinous, pellucid 

 creature of a pale blue colour, with a scarlet mouth and 

 extremity. Dr. Scoresby says that it occurs in vast 

 numbers in some situ.ations near Spitzbergen. Phipps 

 remarks — " Our fishermen call them by the name of 

 ' Whale-food,' and are of the same opinion with Mar- 

 tens, who says they are the chief food of the whalebone- 

 whale." Martens, in his " Voyage into Spitzbergen," 

 tells us that they are called the " Sea May-fhe." " I am 

 of opinion," he says, " that the birds feed upon them. 



because the lumhs, pigeon-divers, and 2'nt'ret divers are 

 plentifully seen in those places where these fisli or sea- 

 insects are seen." 



Family— PNEUMODERMIDiE. 



The Pneumodermons have a fusiform body, and the 

 head is provided with arms furnished with pedicelled 

 suckers. The wings or fins are rounded, entire, with a 

 central foot-like appendage placed at the base of the 

 head. The gills are placed on the hinder part of the 

 body. There are four genera in this familj', each distin- 

 guished from the other chiefly by the gills. 



Ti£E Genus Pneumodermon has them in the form 

 of a four-lobed leaf at the extremity of the body. 



Spongiobranchia has them in the form of a pro- 

 minent spongy ring near the end of the body. 



Tkichocyclus has them in the form of a ciliated 

 ring round the middle of the body, one round the 

 base of the head, and another on the hinder end of the 

 body. 



Family— CYMODOCEID^. 



The Cymodoceas have the body divided into two 



parts, and are furnished with four wings or fins, the 

 two upper broad and rounded, the lower nearly linear. 

 The family contains one genus, Cymodocea, and this 

 genus only one species, C. diaphana — a diaiihanous, 

 translucent, little creature, with the scarlet viscera 

 shining through it. It is found in the Alantic Ocean. 



Class IV.— CONCHIFERA (Bivalves). 



The Bivalves come next to the Univalves in variety 

 and importance ; and though in the number of species 

 they are inferior, they exceed them in the number of 

 individuals. They are all aquatic, the greater propor- 

 tion being marine, and are widely diffused over the 

 globe, ranging from low water mark to a depth of more 

 than two hundred fiithoms. The animals have no dis- 

 tinct head — hence called by many naturalists Acepluda, 

 or headless — no tentacles,and (inbyfar the greater num- 

 ber at least) no eyes. The mouth is concealed between 

 the folds of their mantle ; it is situated near the front of 

 the base of the foot, and is provided laterally with two 

 pairs of elongate fleshy lips, forming lamellar ^wZ/m. It 

 possesses no teeth, and the creatures thus can only 



seize upon such particles as the water floats within 

 their reach, or which are brought near the mouth by 

 the currents that are continually circulating within the 

 cavity of the mantle. These currents not only serve, 

 but supply wafer to the gills for aerating the blood ; 

 they are produced by the action of a number of cilia 

 that clothe the gills. The foot of the Bivalves has 

 some analogy with that organ in the Gasteropods, being 

 formed by the under part of the body, but is not so 

 well adapted for locomotion as in these latter molliisks. 

 Instead of forming a flattened disc as in them, it is 

 generally laterally compressed and keeled — (see fig. 

 228, Venus verrucosa) — though indeed there are many 

 exceptions to this rule. In the Oyster and Scallop 



