402 CHARACTERS OF INVERTEBRATE ANIMALS 



Lastly, it may be noted that the eyes of Peripatus agree more 

 closely with those of Annelids than with those of Arthropods. 



B. AQUATIC ARTHROPODS (BRANCHIATA) 



The four Arthropod classes which have so far been reviewed, 

 i.e. Insects, Arachnids, Myriapods, and Prototracheates, together 

 make up the air-breathing or Tracheate division of Arthropoda ; 

 and we now come to the aquatic division, including the two classes 

 of Crustaceans (Crustacea) and King-Crabs (Xiphosura), to which 

 the Sea- Spiders (Pycnogonida) are doubtfally appended. 



CLASS 5. CRUSTACEANS (CRUSTACEA) 



This very large class, of which a typical member, the Lobster, 

 has already been partly described in contrasting Vertebrates with 

 higher Invertebrates (p. 302), includes animals which are for the 

 most part marine, though many, of the minute forms especially, 

 inhabit fresh water, and some few are terrestrial. 



So great is the diversity of structure within the limits of the 

 class that no single type fully illustrates it; but it may be as 

 well to enlarge somewhat upon the description already given 

 of the Lobster (Homarus vulgaris), taking it as a good average 

 example of the higher Crustacea. To those desirous of seeing 

 how a single type may be made to illustrate the whole of the 

 class in question, and at the same time give a sound knowledge 

 of the principles of zoology generally, a perusal of Huxley's classic 

 work, The Crayfish, is recommended. 



External Characters of the Lobster (fig. 248). The body is 

 bilaterally symmetrical, and the hinder part of it, or tail, is clearly 

 divided into segments. The front part of the body is a cephalo- 

 thorax, consisting of head and thorax closely fused together, 

 though segments are present here too, as shown by the 

 numerous paired appendages. The head is marked off from 

 the thorax by means of a distinct groove (cervical groove). 

 The number of segments appear to be as follows : head, 5 ; 

 thorax, 8 ; tail or abdomen, 7. All the segments, and the ap- 

 pendages they bear, are constructed on the same common plan 

 (see p. 195), but there are many differences in detail, to serve 

 various physiological ends. 



