THE PYCNOGONIDS. 41 



pair of diverticula from the gut runs forward into the limb as far 

 as the glands of the mandibles, which are still retained. 



Where these pouches join the digestive tract a second pair of 

 diverticula originate and passes to the base of the third pair of 

 appendages. A large pair of diverticula run out into the penul- 

 timate segment of the fourth pair of appendages, and behind 

 these is a pair of pouches, the fourth, into the fifth pair of appen- 

 dages, and, lastly, traces of a fifth pair of pouches is found. 

 In Plate VIII, Fig. 52, there is a cross-section through the pos- 

 terior part of this larva. It passes through the fifth pair of 

 appendages, which are still at the sides of the body, and below 

 the section passes through a pair of small forming ganglia. This 

 section shows the thick ectoblast, the scattered mesoblast cells, 

 especially abundant in the legs, and the large endodermal cells 

 filled with food particles. On each side of the ventral ganglia 

 are two other invaginations of the ectoderm. These push in 

 both above (not seen in this section) and below and from behind 

 — forwards as well, and form a partition wall in the body cavity, 

 which cuts off a pair of appendages. The invagination for the 

 ventral ganglia, shown in the figure on each side of the ventral 

 median line, are the ingrowths for the fifth and last pair of body 

 ganglia. There is a distinct proctodeum opening behind. In a 

 single section of this series I found a well-marked 'anlage' of 

 the sexual organs. It was on the dorsal surface of the digestive 

 tract, in the middle line, and in that part of the body corres- 

 ponding to the base of the first pair of walking legs. It was 

 thickest in the middle, and tapered away towards the sides of the 

 larva, but was confined entirely to the body. Dorsal to this 

 mass of cells of the reproductive organ was what seemed to be an 

 accumulation of blood corpuscles, and I fancied I could make 

 out traces of a large heart cavity. In another embryo of the 

 same age as the last I could find no traces of reproductive 

 organs. There are four eyes on the dorsal surface, and the first 

 pair are nearer together than the posterior pair, and on each side 

 of the first pair there is a thickened mass of ectoblast cells. We 

 saw there were five pairs of ventral ganglia, and it is worthy of 

 note that the embryo with a single pair of walking legs has formed 

 the ' anlage ' of all the other ventral ganglia to the appendages. 



In stage 6, represented by Fig. XVI, we find a second pair 



