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ON THE SIGNIFICANCE OF THE SOMITIC CONSTITUTION, 
BODY FOEM, AND GENITAL APERTUEES IN THE 
HIEUDINEA, IN EEFEEENCE TO THE AETHEOPODA. 
By E. J. GoDDAKD, B.A., D.Sc, Zoology Department, Stellenbosch, S.A. 
(Finally received December 6, 1913.) 
The main object of this paper is to detect a meaning in the constancy 
of the somitic constitution, and in the position of the genital apertures of 
the Hirudinea. 
Since writing the original description of the genus Semilageneta the 
author has been much exercised over its peculiarity in regard to the body 
abbreviation. With the exception of this genus and Acanthobdella the 
Class Hirudinea is wonderfully compact considering the variety of habitat 
and media. Acanthobdella was more easily eliminated from the field of 
investigation owing to markedly intermediate characters as linking up 
Hirudinean and Chaetopodan morphology — or rather Oligochaetan mor- 
phology. Thus the unique position of Semilageneta was accentuated, for 
Acanthobdella shows much less aberration in regard to the external body 
form, and as the rest of the class is so compact and so constant in regard 
to the somitic constitution inquiry was stimulated towards the explanation 
of this peculiar position. 
The results embodied in this paper, if correct, may merit importance, 
since, while explaining the external body form of Semilageneta and re- 
establishing understanding and uniformity within the Class Hirudinea 
they serve to explain the significance of this apparent aberration. 
Did no such form as Semilageneta exist, the suggestions offered might 
not, perhaps, entice serious consideration ; but as in so many other groups, 
a single genus has given the clue. Such cannot be dismissed at once as 
ultra-speculative, but merit at least some consideration from the fact that, 
starting from Semilageneta, we find a series of gradations in regard to the 
body form right through the class. Further, they serve, on extra-Arthro- 
podon grounds, to uphold the continuity and " naturalness " of the phylum 
Arthropoda, and to link that phylum more closely to existing Annulata. 
