232 AETHUR WILLEY. 



the mouth has bent round the anterior extremity of the alimentary cauaL 

 There are seven secondary slits, the seventh not yet perforated ; the first is 

 still above and between the third and fourth primary slits. Notice the 

 flattened tops of the larger secondary slits. All the primary slits behind and 

 including the fourth now bend under the pharynx towards the left side. Notice 

 the endostyle and club-shaped gland. 



Fig. 5. — Stage IV. Right side. Thirteen primary slits, the thirteenth 

 closing. Seven secondary slits, the first and last having only just opened. 

 The true first secondary slit has now appeared, and lies above and between 

 the second and third primary slits. The larger secondary slits have begun to 

 double in to form the tongue-bars. Notice the direction of the long axis of 

 the secondary slits. All the primary slits are now bending under the pharynx, 

 and the first is extremely small. The endostyle has definitely commenced to 

 grow beyond the club-shaped gland. This should be noticed carefully and 

 compared with the condition of the two structures in the preceding stages. 

 The atrium is quite closed in front. 



Fig. 6. — Stage IV. Left side. This is to show the extent to which the 

 primary slits have come round to the left side at this stage. The number — 

 fourteen — of primary slits is not typical for this stage (see Fig. 5). The right 

 or anterior half of the velum is seen with a deep focus. There are six 

 elements of the buccal cirri in the lower half of the oral hood, growing out 

 into tentacles and covering up the external orifice of the club-shaped gland. 

 The upper half of the oral hood is hanging over the upper wall of the mouth 

 or velum and over the preeoral pit. 



Fig. 7. — Stage V. Right side. This is the very commencement of Stage V; 

 and the first primary slit, though very small, is still to be seen in side view. 

 The secondary slits, of which there are eigiit, have become larger ; and in the 

 third slit the tongue -bar has actually fused with the ventral wall of the slit. 

 The endostyle and club-shaped gland have the same relations as in Fig. 5, 

 The caecum is commencing to grow out; there is no fixed stage at which this 

 occurs. The primary slits have nearly disappeared from this side, but are still 

 seen below the secondary slits. 



Fig. 8. — Stage V. Left side. The primary slits are not fully round to the 

 left side yet, but the larger of them are commencing to form tongue-bars. 

 The twelfth primary slit is very small, and the thirteenth and fourteenth are 

 closing. Notice the point of junction of the upper and lower portions of the 

 oral hood ; the latter is prolonged anteriorly as a ridge. The right wall of the 

 velum is deeply focussed. 



Fig. 9. — Stage V. Three-quarter ventral view. This shows the first 

 primary slit in a condition of approaching atrophy. The twelfth is also closing. 

 The bases of the secondary slits can just be seen. 



Fig. 10. — Stage V. Ventral view. This should attract particular attention. 



