5iO 



COMPAEATIVE ANATOMY. 



part of the body. This space is gradually reduced in size in the 

 Craniota ; it still, however, retains its respiratory function, but 

 many other organs are also differentiated in it ; these are, largely, 

 accessory organs for the ingestion of food. 



Respiratory Ante-chamber (Cephalic enteron). 



§405. 



In Amphioxus this portion is bounded, in its most anterior 

 region, which is close to the cavity which carries the mouth, by a 

 ciliated apparatus ; there are a number of movable processes also 

 at that point, which are directed towards the lumen of the tube, 

 and so prevent the entrance of foreign bodies. The ante-chamber 

 (Fig. 303, d), which occupies about two-fifths of the whole length 

 of the body, has its walls broken through by a large number of 

 obliquely-set clefts ; these form a complicated framework, the sup- 

 ports of which have been already (§ 353) mentioned. The water, 

 which is taken in by the mouth (a), passes through the clefts, and so 

 to the exterior. But as two lateral dermal folds are gradually con- 

 tinued ventrally over the surfaces on which the clefts are placed, and 

 become united below, a peribranchial cavity is formed, which opens 

 by a special pore (c). It should here be remembered that there was 

 something similar to this in the Ascidi^ (§ 310). But it would not 

 be correct to suppose that the two structures are morphologically 

 identical. A vascular plexus is distributed in the walls of the clefts, 

 the water that streams past effects respiration, the clefts function as 

 branchial clefts, and the whole cavity represents functionally a 

 branchial cavity. 



There are many special points in this arrangement in Amphioxus, 

 such as the want of symmetry in the branchial frame, and its inde- 



Fig. 304. Vertical median section of a larva of Petromyzon. n Mouth, v Velum. 

 h Hypobranctial groove, n Spinal chord, ch Notochord. a Otocyst. c Heart 



(after a di-awing by Calberla) . 



pendence of the metamerism of the body; so that there is altogether 

 a great difference between this apparatus and that of the Craniota. 



