266 ANIMAL PHYSIOLOGY. [LESSON 82. 



the siphon (c) on the left side of the figure. On opening the an- 

 terior part of the trunk the retracted proboscis (</), with its muscles 

 (h), are seen extending backwards to the right of the brain (i), which 

 rests on the inferior turn of the oesophagus. From this cerebral 

 mass (i) large nerves are seen extending forwards to the head (b), 

 the tentacula, with the eyes, at their base (s, s), and to the broad 

 fin-like anterior fold (a) of the long tapering foot. Other nervous 

 chords are seen extending downwards to the ventral surface of the 

 abdomen, and backwards to the sympathetics which supply the ab- 

 dominal viscera. 



1198. This gradual concentration of the ganglionic matter of the 

 great O3sophageal nervous ring of the Gasteropods into a cephalic 

 position, may be regarded as preliminary to its enclosure in a distinct 

 cranial covering a condition which it attains in the Cephalopoda 

 (kephale, head ; pous, foot). 



LESSON LXXXI. 



NEKVOUS SYSTEM IN THE PTEEOPODA AND CEPHALOPODA. 



1199. These animals are named from the Greek, pteron, a wing, 

 and pous, a foot, and are so called from the peculiar wing-like form 

 of their sole locomotive organs, which are placed near the head. 



1200. All the animals of this class are minute, the largest (Clio 

 borealis) not exceeding an inch in length. They are marine, and 

 found for the most part in the Arctic seas, where they exist in im- 

 mense shoals. Some of them are encased in an extremely delicate 

 shell (Hyalea) ; others, like the Clio, are naked. They constitute, 

 especially (7. borealis, the chief food of the Whale, which is unable to 

 swallow any thing of larger size than the creatures now under con- 

 sideration ; and of these, from the smallness of its oesophagus, it can 

 only take one at a time. 



1201. The nervous system in the Pteropods presents the same 

 general plan, and the same varieties of form in its cephalic masses, as 

 we have seen existing in the Gasteropods, especially in the naked species. 



1202. Thus, in the Clio borealis, there is a double nervous collar 

 around the oesophagus ; two small ganglia approximate to each other, 

 to form a bilobate brain (a) (Fig. 382), and are placed above the 

 oesophagus, immediately behind the lips, and indicate by their di- 

 minished size the imperfect development of the organs of the senses 

 in this animal, which scarcely presents a trace either of eyes or ten- 



