CHAPTER XXXIll 



ANIMAL RESPIRATION— NEMERTINES— MOLLUSCS 

 WHICH BREATHE IN WATER 



NEMERTINES 



Some authorities are of the opinion that these curious un- 

 segmented marine creatures, which look like long cylindrical 

 worms (see vol. i, p. 305), resemble in certain respects the re- 

 mote stock which has given origin to backboned animals. One 

 characteristic feature of such animals is, as we have seen, the 

 possession of a perforated pharynx which has taken on the 

 function of breathing. Nemertines in all probability breathe to 

 a large extent by means of their soft skin, but it is also likely 

 that the pharynx takes part in the same work, although it is 

 not perforated by gill-clefts. And it is certainly a suggestive 

 fact that the straight digestive tube of these creatures bulges 

 out on each side into a series of pouches, resembling in character 

 the structures which probably gave rise to gill-pouches by acquiring 

 external apertures (see p. 381). 



MOLLUSCS WHICH BREATHE IN WATER 



The vast majority of Molluscs of all kinds live in water, 

 and breathe the oxygen which is dissolved in it. They are 

 predominatingly marine, but some forms live in estuaries, while 

 others have migrated into fresh water, and adapted themselves 

 to the special conditions there existing. 



MAIL-SHELLS, &c. (Protomollusca) 



The breathing organs are in a comparatively simple condition 

 in the primitive forms known as Mail- Shells (Chiton, &c.). Here 

 there is a flattened oval body, protected above by a row of eight 



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