356 PALAEOSTRACA. 



4. General Considerations. 



We have already (p. 352) pointed out that the Xiphosura are 

 evidently somewhat closely related to the Trilobites. Certain 

 characters which tend to connect the two are specially apparent 

 in the young larvae of Limulus (Trilobite stage), their presence in 

 the adult also being undeniable. Here, as in the Trilobites, the 

 anterior region of the body is divided by two longitudinal furrows 

 into a middle and two lateral portions. The position of the lateral 

 eyes agrees in the two groups, but the occurrence of ocelli (median 

 eyes) has not yet been fully established in the Trilobites. The two 

 groups further agree in the general configuration of the cephalo-thorax, 

 in the bending under of the anterior part of the cephalic shield, and 

 in other points. Our present knowledge of the cephalic limbs of the 

 Trilobites seems to indicate that they resembled in structure those 

 of the Gigantostraca.* Four pairs of jaw-feet have been found in 

 the latter, the last pair having been specially developed, and having 

 grown out into broad, oar-like limbs. Various fossil forms of 

 Xiphosura connect Limulus with the Trilobites ; among these, 

 Belinurus, by the shape of its cephalo-thoracic (cephalic) shield, 

 which is continued into long cheek -spines, strikingly recalls the 

 Trilobite family, Trinudeidae. 



Although the Palaeostraca thus appear to form a single group 

 based upon natural relationship, a certain distant relationship to 

 the Crustacea is, as we have already pointed out (p. 315), undeniable. 

 This is supported chiefly by the structure of the Trilobite limbs, 

 described more in detail by WALCOTT (No. 5),* and by the biramose 

 character of the abdominal limbs of Limulus. We have already 

 stated why we regard the Palaeostraca and the Crustacea as groups 

 of equivalent value, but have abstained from uniting them. We, 

 however, think ourselves justified in assuming that they both had 

 their origin in a common racial group, the Protostraca, which may 

 perhaps also be regarded as the racial group of the Onychophora 

 Myriopoda series. 



The relationship which appears to exist between the Palaeostraca 

 and the air-breathing Arachnida deserves closer consideration. As 

 early as 1829, STRAUSS-DURKHEIM emphasised the near relationship 

 between Limulus and the Arachnida. He based his view chiefly 

 on the radial arrangement of the limbs, on the common sternal plate, 

 and on the presence of an inner endoskeleton (the endosternum), 

 * See Editorial note, pp. 333 and 360. 



