MUSEUM OF COMPARATIVE ZOOLOGY. 209 
adopted in accordance with the advanced views of Milne-Edwards, 
Gegenbaur, Lankester, Van Beneden, Verrill, and other authors. 
The more conservative zoülogists consider the group as an order or 
sub-class of the Crustacea, while others view it as a sub-class of the 
Arachnida.* Professors A. Milne-Edwards,] Gegenbaur,f and E. Ver- 
rill 8 consider the group as a class of the Arthropoda, placing it after the 
Crustacea and preceding tbe Arachnida in the scheme of classification. 
With this course we are in accord. | 
The following arrangement is made to express the view of the rela- 
tions of the different orders forming the group. 
ARTHROPODA. 
Crass PŒCILOPODA. 
Sub-class MEROSTOMATA. Sub-class PALEADA. 
Order Xiphosura. Order Trilobita. 
Order Hurypterida. 
PŒCILOPODA. Arthropods with the cephalic appendages sub- 
serving the function of manducation. 
Sub-class MEROSTOMATA. Peecilopods with ocelli in addition to 
compound eyes, all the limbs serving as mouth organs, the mouth pro- 
vided posteriorly with a metastoma. 
Order Xiphosura. Mouth furnished with a small hypostoma and 
six pairs of appendages. Posterior segments of the body more or less 
free, and all bearing branchiæ or reproductive organs. 
Order Zurypterida. Mouth furnished with five pairs of appendages. 
Two anterior free segments, bearing branchiæ or reproductive organs. 
Other free segments devoid of appendages. 
Sub-class PAzæADæ.  Poecilopods with numerous thoracico-abdomi- 
nal appendages. Eyes compound (when developed). Ocelli unknown. 
* Professors Ed. Van Beneden, E. Ray Lankester, Introduction to Gegenbaur's 
Elements Comp. Anat., English ed., 1878. 
+ Ann. des Sci., XVII., 1872. 
+ Elements Comp. Anat., English ed., p. 230, 1878. 
§ Classification of Animals, Yale College, 1879. 
| “It is by no means desirable that students should be taught to accept any one 
scheme of classification as finite. They should be taught to look upon these schemes 
as the condensed expression of an author's views, — as the epitome of his teaching, 
facilitating the recollection and comparison of conflicting solutions of the vast series 
of unsolved problems of morphology.” Prof. K, Ray Lankester, 
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