iv. 17 FEEDING OF AMMOCOETE LARVA 117 



Feeding takes place by the intake of water through the mouth and 

 the separation of small food particles from it in the pharynx (Fig. 77). 

 For this purpose there is used a great quantity of mucus, which is 

 secreted by the endostyle and gathered into a strand by the cilia of 

 the pharynx. This endostyle is a most remarkable organ, forming early 

 in development as a sac below the pharynx (Fig. 78). It consists of a 

 pair of tubes, on the floor of which there are four rows of secretory 

 cells (Fig. 79). There is a single opening to the pharynx, by a slit 

 at about the middle of the length. As development proceeds the inner 

 rows of cells at the hind part of the organ become coiled upwards, and 

 at the end of larval life the endostyle therefore forms a very large mass 

 below the pharynx, composed of tubes lined partly by secretory and 

 partly by ciliated cells. Probably no enzymes are secreted by the 

 endostyle, its function being to produce mucus in which the food 

 particles become entangled. Although it resembles the endostyle of 

 amphioxus in the arrangement of the secretory columns, there is a 

 difference in that the organ in the ammocoete larva is not an open 

 groove. There is, however, a ciliated groove in the floor of the pharynx, 

 that is to say, on the roof of the endostyle (Fig. 79). 



The details of the feeding-currents of the ammocoete larva are not 

 understood. An important difference from the arrangement in am- 

 phioxus is that the current is produced by muscular rather than ciliary 

 action. The velum, a pair of muscular flaps, provides the main current 

 when the animal is at rest. The branchial basket can also be expanded 

 and contracted by an elaborate system of muscles. It is not easy to 

 observe how the food particles are taken up from the current, but 

 apparently a strand of mucus shoots from the endostyle and occupies 

 the whole of the centre of the pharynx (Fig. 77). This strand probably 

 rotates and as it passes backwards into the eosophagus it catches the 

 particles. 



Evidently the system enables the animals to feed efficiently on the 

 small unicellular algae and bacteria of the mud. In amphioxus the 

 ciliated pharynx, occupying a considerable proportion of the whole 

 surface, is only able to support a tiny creature, but the muscular 

 feeding-system of the ammocoete allows a relatively small pharynx to 

 feed a fish 170 mm long and weighing up to 10 grams. This use of 

 muscles for moving the gills was evidently an important step in 

 chordate evolution. It allowed the animals to escape from the limita- 

 tion of size imposed by the ciliary method of feeding. After the 

 development of jaws to form a still more efficient feeding mechanism 

 the rhythmic movement of the branchial apparatus persisted for the 



