MOLLUSCS. 



359 



the old woman in her shoe, paddling about by the large oval wings ; but, unlike the 

 other shelled pteropods, it has not the power to retract itself within the aperture. 

 Two genera are known ; CymbuUa, with a slipper-shaped shell pointed in front and 

 square behind, and Tiedemannia, with a body much like that of CymbuUa, but with 

 the shell internal. Tiedemannia can change its colors at will by means of chromato- 

 phores much like those of the ceplialopods soon to be described. 



Oeder II. — GYMNOSOMATA. 



The Gymnosomata are naked pteropods, in which the head is distinct and well 

 separated from the body and the foot, and in which well-developed tentacles are 

 present. The wings are distinct from 

 the foot, and external gills are present 

 in one family. The young at first are 

 provided with a shell and swim by means 

 of a velum, but soon both these embry- 

 onic structures are lost, and, pending 

 the development of the wings, the larva 

 swims by means of bands of cilia which 

 surround the body, as shown in our figure 

 of the larva of Pneumodermon. 



In the Clionid^ the body is spindle- 

 shaped, and the head bears tentacles 

 which are without suckers. In Clione 

 there are two of these tentacles, and, 

 besides, from either side of the mouth 

 arise three pairs of extensible processes 

 armed with minute suckers. The num- 

 ber of these suckers is enormous. In 

 the fresh specimen the appendages are 

 reddish in color, but under a low j^ower 

 of the microscope the color is seen to 

 be due to a number (about three thou- 

 sand on each tentacle) of red cylindrical 

 sheaths, and upon the use of a higher 

 amplification each of these cylinders is 

 seen to contain about twenty suckers placed on slender stems, and capable of being 

 protruded from the mouth of the sheath. Now, since a sheath contains twenty 

 suckers, and since there are about three thousand sheaths on each appendage, a 

 simple mathematical operation shows us that each Clione is furnished with about 

 360,000 suckers. 



Clione borealis, as its name implies, is an Arctic form which is frequently seen in 

 vast patches. Together with Limacina borealis, a species belonging to the last order, 

 it furnishes the principal food supply of the whalebone whales. The whalers call it 

 ' brit,' and the presence of one of these schools of brit is considered as indicative of 

 a good whaling ground. Clione papilionacea occasionally occurs on the eastern coasts 

 of the United States, and has been reported as far south as New York. It may be 

 that its apparent rarity is due to the facts that it is a northern form, and that natural- 



Fio. 470. — Larva o£ Pneumodennoji. 



