382 ANIMAL RESPIRATION 



part of an aquatic animal to which water has access may help in 

 breathing if its covering or lining is sufficiently thin for the oxygen 

 dissolved in the water to diffuse in, and the carbonic acid of the 

 blood to diffuse out. The lining of the digestive tube is commonly 

 sufficiently delicate to satisfy this condition, and since an aquatic 

 animal takes in more or less water with its solid food, it therefore 

 follows that in many marine worms, &c., breathing is to some 

 extent performed by the tube in question. It is also obvious that 

 the beginning of the digestive canal is most favourably situated 

 for the purpose, and it is therefore not surprising to find that 

 aquatic breathing has in many instances been localized in the 

 pharynx. A breathing organ, to be efficient, must possess a suffi- 

 ciently large surface for exchange of gases between the blood and 

 the surrounding medium ; hence any folds or pouches in the sides 

 of the pharynx would add to its utility in this particular direction. 

 And here the very pertinent question naturally presents itself — 

 "Is there any reason for thinking that the lowly and long-extinct 

 creatures from which Vertebrates have sprung possessed a pouched 

 digestive tube?" This question may be, with some probability, 

 answered in the affirmative, for Vertebrates are segmented animals 

 (see p. 375), i.e. are divided from before backwards into a series 

 of parts or segments which, in such a low type as the Lancelet 

 (Amphioxus), resemble one another pretty closely. The original 

 purpose of segmentation appears to have been lost in Vertebrates 

 (see p. 375), and it is to be looked upon as an inherited character 

 which they have had to make the best of. If we examine some 

 of the segmented Invertebrates, especially the segmented worms 

 (Annelids), we shall find that the digestive tube often bulges out 

 laterally in each segment, and this is the very feature which might 

 lead to the formation of gill-pouches in the region of the pharynx. 

 How and why these pouches first came to open externally is a 

 very difficult problem, but we know that such structures may com- 

 municate with the exterior, for in some of the Sea-Slugs (Nudi- 

 branchs, see p. 357) there are tubular branches of the digestive 

 tube which open upon horn-like projections {ceratd), probably for 

 the purpose of getting rid of certain waste products. A similar 

 reason may explain the origin of gill-slits in the remote past. Be 

 that as it may, such openings once established would greatly 

 increase the efficiency of the pharynx as a breathing organ, for 

 they would afford a means of exit for water taken in at the mouth, 



