24 THE ANATOMY AND DEVELOPMENT OF PERIPATUS NOVAE-BRITANNIAE. 



In this stage the meso-somatic wall of the somite is thicker thau the meso-splanchnic 

 wall. In the latter there are often relatively wide intervals between the scattered nuclei 

 whereas they are always compact and often many-layered in the somatic wall. In the 

 first somite, however, the mesodermic layer is uniformly thick, the nuclei occurring 

 throughout in a single row. 



Stage IX., Fig. 33. This is the stage at which the embryo is coiled upon itself 

 spirally. I have seen other embryos of approximately the same age as this which were 

 not spirally coiled but merely flexed, and it may be stated that every embryo does not 

 necessarily pass through a stage in which it is coiled exactly in this manner (Fig. 33 a). 

 The caudal extremity of the body has now grown to such an extent that it has 

 come to lie in front of the head. The cephalic end of the embryo has maintained its 

 primitive position, and there is, as yet, no cephalic flexure but only caudal and 

 abdominal flexures. A true cephalic flexure is met with for the first time in the next 

 stage. The antennae have now made their appearance as outgrowths from the cephalic 

 lobes, or to speak perhaps more correctly, the cerebral ganglia have become differentiated 

 from the ectodermal thickenings at the bases of the cephalic lobes while the antennary 

 portions of the lobes have increased in length and independence. This is the stage 

 during which the lips which enclose the 2nd pair of appendages — the manducatory 

 appendages — are formed (Fig. 37). The eye-vesicles are also present. The optic groove 

 was present in the preceding stage (Fig. 59). 



Botation of Stomodoeum. The stomodoeum no longer extends straight forwards 

 but is directed dorsalwards. In still later embryos the stomodoeum is seen to project as 

 a stout funnel-like tube backwards and somewhat dorsally from the mouth (cf. Fig. 36). 

 In the present stage it stretches dorsally and somewhat anteriorly from the mouth and 

 is now best seen from the dorsal aspect of the animal. In earlier stages, as we have 

 seen, it was directed straight forwards. This stage of its development is therefore inter- 

 mediate between its primary forward direction and its secondary definitive backward 

 direction. Thus, in effect, the stomodoeum is rotated through 180°. In the later stages 

 it is best seen from the lateral aspect. A similar rotation, the result of differential 

 growth, has been described by Gaffron in connection with the development of the female 

 generative organs of P. edivardsii. The following is the passage referred to in the second 

 part of Gaffron's work on the anatomy and histology of Peripatus (6, p. 147): — "Bei 



einem Embryo von 1*8 cm. Lange ... findet man vor Allem, dass das Ovarium seine 



Lage um 180° geandert hat, indem es jetzt von seinem Befestigungsort nicht mehr nach 

 hinten, sondern nach vorn gerichtet ist." (See Gaffron, loc. cit. Taf. XXI., Figs. 1 and 2.) 



Such instances as these of the ontogenetic transposition of parts are probably of 

 some importance. It is at least a remarkable fact that the stomodoeum of P. novae- 

 britanniae occurs at first as a praeoral tube and is later transposed into a post-oral tube. 

 This is not a mere playing with words, because, what is at first the anterior extremity 

 of the stomodoeum becomes, after the transposition has been effected, its posterior 

 extremity. 



Stage X. (PI. Ill, Figs. 35 and 36). In this stage the relative dimensions of the 

 trophic organ and embryo have undergone a considerable change, and were it not for the 

 remarkable procephalic prolongation of the vesicle which is still present in Figure 35. 



