The Development of Theridmm. 313 



jointed. The abdomen possesses eight segments (Ah. 1-Ab. 8, Figs. 

 63, 66) anterior to the caudal lobe (CaucL), the full number that it 

 will have in this species ; and of these the second to the fifth inclusive 

 bear each a pair of limb buds (Ah. 2h-Ah. 5h). For each of the 

 thoracal segments (Fig. 65) and to each of the abdominal except the 

 eighth (Figs. 63, 66) there is a pair of nerve ganglia. 



On the head lobe appears the stomodaeum (Sto., Figs. 64, 65), 

 a shallow pit with somewhat turgid lips (8to. L.). It is an ecto- 

 blastic invagination shown on median section in Fig. 67b, PL VI, 

 and on cross section in Fig. 68b. Just at its anterior border is a pair 

 of small, basally contiguous prominences, the rostral appendages 

 (Eos., Figs. 64, 65, PI. V) ; later these will fuse to form the rostrum. 

 They are shown on longitudinal section in Fig. 67b, PI. VI, and on 

 transverse section in Figs. 68a and 68d. Beneath these appendages 

 lie the rostral mesoblast sacs, which occupy more than the anterior 

 half of the head lobe, meet in the midline anterior to the ventral 

 sulcus, and are continued in the lateral lips of the stomodaeum. 

 The upper portion of Fig. 68d shows how these sacs extend laterally 

 almost as far as the head lobe itself; Fig. 67b shows that they extend 

 mesially back to the posterior border of the stomodaeum; and Fig. 

 67a shows one rostral sac on longitudinal section in the plane of a 

 cheliceron (CJiel.), this demonstrating how much more extensive the 

 rostral sacs are than the cheliccral. Each rostral sac now consists of 

 somatic and splanchnic layers, and these layers separate from each 

 other to form coelomic spaces beneath the rostral appendages (Ros.. 

 Figs. 67b, 68a and 68d) at the lateral margins of the head lobe 

 (R. Coe I. J, Figs. 68c-f), and beneath the cerebral ridges (Ce. IL, 

 Figs. 67a, 68f). The only prominences of the head lobe anterior to 

 the chelicera that can be properly considered prestomial appendages 

 are these rostral tubercles ; and they may be rightly adjudged cephalic 

 appendages to which belong the rostral mesoblast sacs, and the 

 ganglia of which would be the cerebral. Though they develop later 

 than the chelicera it will be recalled that the chelicera arise later than 

 the pedipalps and the legs, and the rostral mesoblast sacs develop 

 simultaneously with the cheliceral. Fusion of these rostral append- 

 ages with the lips of the stomodaeiun follows later, the two are 



