THE ANATOMY AND DEVELOPMENT OF PERIPATUS XOVAE-BRITANXIAE. 17 



extremel} - thick, about as thick, in fact, as the pygidial bulbus of our species; and the 

 two narrow accessory glands enter the muscular mass of the ductus in the Cape species, 

 exactly as the ducts of the pygidial glands enter the bulbus in P. novae-britanuiae. 

 We have here, therefore, an interesting example of compensating growth. 



In P. novae-britanniae the external opening of these glands leads into a narrow 

 tube with smooth epithelial lining and chitinous intima. After the median tube has 

 divided and the paired ducts emerge from the bulbus, the lumen soon increases slightly 

 in diameter. At the point where the ectodermal portion of the tube is continued into 

 the mesodermal portion 1 , the lumen becomes suddenly narrowed and the intima ceases. 

 But this constriction is not visible externally because the muscular coat becomes pro- 

 portionately thicker in this region. The enlarged muscular coat and the reduced lumen 

 continue for a short distance and then the lumen gradually enlarges pari passu with a 

 diminution in the thickness of the tunica muscularis. Finally, the anterior portion of 

 the gland appears in section as a thin-walled tube with very wide lumen, lined by a 

 well-marked smooth epithelium. 



The preceding account of the finer anatomy of the pygidial glands (apart from 

 the highly characteristic bulbus) differs from Gaffron's description of the anal glands of 

 P. edwardsii chiefly in the fact that in the latter, the external aperture of each gland 

 leads into a wide chamber with folded walls, presenting the same appearance as the 

 rectum itself. So that they are well called anal glands and I think it is advisable to 

 give separate names to structures, even though obviously homologous, when they have 

 such very different anatomical relations. 



Kennel (11, Pt. II, p. 70) has shown that in the Neotropical species whose develop- 

 ment was studied by him, the anal glands are the modified nephridia of the apodal 

 anal segment. This fact is confirmed by the position of the openings of the corre- 

 sponding glands in P. novae-zealandiae outside the nerve-cords (Sheldon). Kennel further 

 states that a rudiment of these glands is laid down in the female embryos and subse- 

 quently undergoes degeneration. 



Why do these glands differ so very much in their manner of discharging to the 

 exterior, in one case opening coincident ly with the anus, in another opening into the 

 ductus ejaculatorius, in another opening independently with paired apertures between 

 generative pore and anus, and in a fourth case opening by a median dorsal aperture? 

 This is no doubt a difficult question to answer, but the fact that such differences do 

 occur is one of considerable interest. For my part, I am tempted to think that these 

 accessory, anal and pygidial glands of Peripatus are capable of throwing light upon the 

 morphological nature of the Malpighian tubules of Insects and some other Arthropods 

 and of providing an explanation of the fact that these structures are sometimes ectodermal 

 and sometimes entodermal. 



1 Gaffron does not use the word " mesodermal " in describing the anterior portion of the anal glands 

 of P. edirardsii, but he described it as "entodermal." I do not know whether he made any mental distiuction 

 between entoderm and endoderm — but in any case Kennel objected strongly to the term. If it were not for 

 the risk of falling foul of the germ-layer theory, I should myself prefer the word "entodermal" not as 

 signifying any relation to the technical term "hypoblast" but in simple contrast to "ectodermal." The 

 mesoderm has not the same value as ectoderm and entoderm as has long been realised by many zoologists— 

 but this is a controversial subject. Certainly Gaffron did not mean " hypoblastic " when he used the term 

 "entodermal." 



w. 3 



