Miscellaneous. 169 



DermalcichidoB, Tyroglyphidte, and Sarcoptidae, show no trace of the 

 organ. Whether it occurs in the more highly organized tracheate 

 families of the Acarida must be ascertained by further observations. 



The interpretation of the unilocular hearts of the Entomostraca 

 and Acarida as secondarily simplitied forms agrees completely with 

 the notion, founded upon many other considerations, tliat the Mites 

 are degraded members of the class Arachnoidea, the starting-point 

 of which must probably be sought in the great Palaeozoic Giganto- 

 straca with their resemblance to the Scorpions, hitherto regarded as 

 Crustacea upon insufficient grounds. From the investigation of the 

 development of living members of the group or nearly related types 

 {Xiphosura, Limuhis), although still imperfect, there seems to be 

 more and more foundation for the opinion that these old Pala?ozoic 

 types are morphologically much more nearly related to the Arach- 

 noidea than to the Crustacea, although they have, in common with 

 the latter, their aquatic habitat and, as a consequence, branchial 

 respiration. Hitherto, evidently, far too much stress has been laid 

 upon this last agreement in the division of the Arthropoda into 

 Branchiata and Tracheata, without taking into consideration that 

 the breathing by air-spaces may have been developed in different 

 ways and at different times in the terrestrial forms, and that con- 

 sequently no primarily decisive morphological value is to be ascribed 

 even to the possession of trachect^. Nevertheless the root of the old 

 Gigantostraca and Xiphosura may have a common origin with the 

 stem-forms of the Crustacea, the Protostraca. In the latter, besides 

 the normal construction of the anterior head, the characteristic 

 J^aitplius-lsiVYa. and the doubled number of pairs of antennae may 

 have prevailed throughout ; while the former led up to a second 

 great series of Arthropoda, characterized morphologically, in the 

 first place, by the smaller development of the anterior head, and tlie 

 disappearance of the pair of members belonging to it, the anterior 

 antennce, and further by the presence of six pairs of limbs on the 

 fore body (cephalothorax), reduced to five pairs, however, in the 

 Merostomata (?inall). Besides these two series of Arthropoda, 

 probably united at the base, we have then, as forming a third series, 

 the Insecta and Myriopoda, for the derivation of which the remark- 

 able Annelid-like Onychophora {Peripatus) appear to be so signi- 

 ficant. 



The characters of the three Arthropod-series would then be as 

 foUow^s : — 



Series I. (Crustacea.) — Two pairs of antennae, the second of 

 which represents the first pair of trunk-members removed forwards. 

 A pair of mandibles as the second pair of trunk-members. Two 

 pairs of maxillae. Variety in the number and structure of the 

 usually numerous pairs of limbs on the middle and hind body. 

 Aquatic habitat and branchial respiration. NaiipUns-larva. 



Series 11. (Gigantostraca, Arachnoidea.) — Absence of the an- 

 terior antennae. One pair of limbs in front of the mouth (? belonging 

 to the first or second body-metamere). Five, or sometimes four, 



Ann. & Mag. N. Hist. Ser. 5. FbZ. xvii. 12 



