1897-98.] Dr Masterman on Ar chimeric Segmentation. 281 
these diverticula by constriction,* and how the mesocoeles and 
metacoeles are separated from each other 
does not appear to have been clearly 
made out. 
Altogether, the anatomical evidence 
is in favour of regarding the Chcetog- 
natha as animals which have retained 
a primitive pelagic habitat and a primi- 
tive archimeric segmentation,! although 
both the anatomy and ontogeny show 
some deviations from the original type, 
in the addition of setae, lateral and tail- 
fins, and sense-organs. It is interesting 
to note that Gourret J describes an 
excretory organ in the head. 
The peculiar backward extension of 
the archenteron, resulting in a division 
of the metacoele into two parts, appears 
to point to a former extension of the 
alimentary canal to the posterior end, 
and agrees with the theoretical explana- 
tion given above, of the fact that in 
the Ar chi- coelomata the metacoele is 
frequently paired. 
Enteropneu'sta ( Burroicing ). 
This group shows to perfection the Fig. 1. —Diagrammatic hori- 
pre-oral protomere, the paired meso- zontal section through one 
meres and metameres (fig. 2). The ° f the Chaztognatha. 
protocoele is essentially ‘ animal ’ and muscular, the metacoeles 
* Butschli, 0. Zeitschrift TViss. Zool., xxiii., 1873. 
t “ If it is permissible to refer the efferent sexual ducts to metamorphosed 
nephridia, we should have to ascribe to Sagittaz at least two trunk somites, 
and accordingly explain the Chaztognatha as forms in which, perhaps in con- 
nection with the manner of locomotion, a primitive segmentation of the body 
has been retained in a degenerated form only.” — Text-BooTc of Embryology , by 
Korschelt and Heider (Translation), p. 371. 
t ZLn. Mus. Nat. Hist., Marseille, ii., 1884-5. 
YOL. XXII. 
T 
