200 THE EVOLUTION OF CONTINUITY 



orifice developing as the anus. In the primitive Cephalopod 

 the inherited orifice developed as the mouth, and it was 

 the anus which appeared as the new alimentary opening, 

 and it communicated with the gill-cleft. It is difficult to 

 say what forces caused the formation of the anus, but no 

 doubt they were pressure forces, possibly the result of 

 peristaltic or contractile movement of the developing gut, 

 and it may be partly due to body-wall contractions, and the 

 outflow of water at the gill-cleft. In any case, the current 

 of the escaping water would doubtless cause the terminal 

 part of the alimentary tube to develop with the anus 

 presenting towards the gill-cleft, the excreta being thus 

 carried outside with the water of expiration. This is what 

 we presume from the anatomy of the present-day Cepha- 

 lopod in the next figure. 



Fig. 66. — Plan of a typical Cephalopod. (After Nicholson.) 

 The notes beneath will help to explain the figure. 



" The body in the Cephalopods is bilaterally symmetrical, and 

 the cephalic region (prosoma) is conspicuously marked out, and is 

 separated from the visceral region (metasoma) which is enclosed in 

 the mantle. The mantle cavity is situated on the under side of the 

 body, when the animal is placed in its natural position. The head 

 is very distinct, bearing a pair of large globular eyes, and having the 

 mouth in its centre. The mouth is surrounded by a circle of eight, 

 ten, or more long muscular processes or arms which are generally 

 provided with rows of stalked or sessile suckers. ... In all the 

 Decapod, and in some of the Octopod forms, the sides of the body 

 are produced into muscular expansions or fins with which the animal 

 swims head foremost. In all the Cephalopods, also, the lateral margins 

 of the foot (' epipodia ') are either placed in apposition (Nautilus) 

 or are actually united (Cuttlefishes) in such a manner as to form a 

 muscular tube known as the ' funnel.' 



