230 COMPARATIVE ANATOMY, 



b) Malacostraca (continued), 



Stomapoda. 



Squilla. 

 Tanaida ). 



Tanais. 



2. Arthrostraca (Hedriophthalma). 



Amphipoda, 



Gammarus, Orchestia, Hyperin, Phrcmyma. 

 Lsemodipoda. 



Caprella, Cyamus. 



I S p cl P. . 



Bopyrus, Cymotlioa, Sphseroma, Oniseus, Asellusj 

 Idothea. 



Limulus. 

 Peripatus. 



II. Pcecilopodat). 



B. Protracheatat). 



C. Tracheata. 



I. Arachnida. 



AutarachnaB§). 



Arthrogastres. 

 G a 1 e o d e a . 



Galeodes. 

 Scorpionea. 



Scorpio. 

 Phrynida. 



Telyphonus, Phrynus. 



Pseud oscorpioncn. 



Chelifer. 

 O p i 1 i o n e a. 



Phalangium, Opilio. 



A ran e a. 



Salticus, Thomisus, Argyroneta, Tegenaria, Mygale. 

 Acarina||). 



Acarus, Argus, Ixodes, Gamasus, Atax, Thrombidium. 



* These form a division which is allied partly to the Thoracostraca, and partly to 

 the Arthrostraca, and which seems to have remained more like the pi'imitive form of 

 the Malacostraca. 



t These are connected with the, palseontologically, very old, and completely lost 

 division of the Trilobita, through the fossil Bolinuridte. There are many characters 

 in their structure and development which point to their being distinct from the 

 Crustacea. 



J More exact investigations into the organisation of Peripatus show that this! 

 animal, which as yet has been generally placed with tho Vermes, is the representative 

 of a special class of Arthropoda, which must be placed before the Tracheata; a much 

 lower stage is seen in it than in any of the larger divisions of the Tracheata. A form 

 seems to be here retained, which separated from tho Tracheate phylum, before it 

 broke up into its several branches. 



§ The true Arachnida vary considerably in tho characters of their body-segments, 

 and in the larger sections formed by the fusion of a number of segments. We 

 regard those forms in which there are several such sections, in which it is still possible 

 to see that they are composed of metameres, as tho least altered and most similar to 

 the primitive form. The Cyphophthalmida) (Gibocellum) and the Chernetidae form 

 small divisions, which belong to the Arthrogastres ; the former are allied to the 

 Opilionea, and the latter to the Pseudoscorpionea. 



| There seems to be no doubt that degeneration is present in these, and is indi- 

 cated by the parasitism which obtains in most of the families ; it leads in the family 

 of the Linguatulida to a very remarkable change in the form of the body. 



