THE ANATOMY AND DEVELOPMENT OF PERIPATUS NOVAE-BRITANNIAE. 23 



(Fig. 65). I think Figure 31 speaks for itself. The enormous trophic organ (tropho- 

 blastic vesicle) which is such a remarkable characteristic of these embryos, is here 

 clearly seen to be a dorsal structure. The rudiments of the appendages are clearly 

 represented. From the preparations it is evident that the thickened ectoderm which 

 takes part in these rudiments also gives rise to the ventral organs from which 

 the nerve-cords are delaminated. This intimate primary union, in such a form as 

 Peripatus, of the appendicular and the neural folds or thickenings, may be a fact of 

 profound physiological meaning. For, presumably, the forefathers of Peripatus were 

 amongst the earliest terrestrial animals to acquire pedal locomotion. When viewed from 

 a purely physiological stand-point one is inevitably reminded of the lateral line of 

 lower Vertebrates and its possible relation to a more or less hypothetical continuous 

 lateral fin-fold or appendicular ridge. 



The complementary functions of locomotion and equilibration 1 combined with the 

 fact of the united origin of nerve-cords and appendages so far as the ectoderm is 

 concerned, may go some way towards explaining or giving a reason for the divarication 

 of the nerve-cords of Peripatus. The old idea held 40 years ago, was, that this 

 indicated a relationship to the Plathelminthes. I think it is safe to say that this view 

 has now a chiefly historical interest. 



It might be inferred, from the double fold in the embryo at this stage, that 

 sections through the middle region would involve three distinct portions of the embryo ; 

 and such is the case, as a glance at Figures 63 — 65 will show. 



The stomodoeum (Figs. 59 — 61) is now present as a long tube opening to the 

 exterior at its posterior end at the base of the cephalic lobes and consequently at the 

 base of the cranial groove which lies between the latter. The stomodoeal tube extends 

 at present straight forwards, below the ectoderm of the cranial groove, and ends blindly 

 at its anterior end. 



This stage is also characterised by the origin of the segmental organ of the 3rd 

 pair of somites (Fig. 62). It arises, as do all the segmental organs, in the hinder 

 somatic mesodermic wall of the somite. It is a tube opening anteriorly into the 

 somite and ending, at present, blindly at the other end. A vestigial segmental organ in 

 the form of a deep pit in the somatic mesoderm occurs also in the second somite but 

 it is not shut off as a tube from the rest of the somite (Fig. 61). No other segmental 

 organs are present at this stage, and I have not attempted to follow their further de- 

 velopment with the limited material at my disposal. If any zoologist should have the 

 opportunity on some future occasion of examining these embryos in the fresh condition, 

 I should recommend him to look for the possible occurrence of cilia in connection with 

 the somatic walls of the somites. The segmental organ of the 3rd somite at this stage 

 looks, in my sections, as if it might be ciliated. The general absence of cilia in the 

 adult Peripatus, except in the male genital ducts and in the ducts of the receptacula 

 seminis in the female where they were discovered by Gaffron whose observation was 

 confirmed by Sedgwick (19), is no doubt connected with the great reduction of the 

 coelom in the adult. 



1 I may be permitted to refer to what I have said on this subject in a former publication (Amphioxus and 

 the Ancestry of the Vertebrates, 1894, p. 42). 



