114 MORPHOLOGICAL DEVELOPMENT. 



somites (or segments corresponding with those of the body) 

 and their appendages, the latter being modified so as to serve 

 the purpose of sensory and manducatory organs." * 



Thus even in the higher Arthropoda, the much greater con- 

 solidation and much greater heterogeneity do not obliterate 

 all evidence of the fact, that the organism is an aggregate of 

 the third order. Comparisons show that it is divisible into a 

 number of proximate units, each of which is akin in certain 

 fundamental traits to its neighbours, and each of which is an 

 aggregate of the second order, in so far as it is an organized 

 combination of those aggregates of the first order which we 

 call morphological units or cells. And that these segments 

 or somites, which make up an annulose animal, were origin- 

 ally aggregates of the second order having independent in- 

 dividualities, is an hypothesis which gathers further support 

 from the contrast between the higher and the lower Arthro- 

 pods, as well as from the contrast between the Arthropods 



* The fusion of the segments forming the Arthropod head and the 

 extreme changes, or perhaps in some cases disappearances, of their appen- 

 dages, put great difficulties in the way of identification ; so that there are 

 differences of opinion respecting the number of included segments. Prof. 

 MacBride writes: — "It is highly probable that a primary head (praeoral 

 lobe or praestomium) has been derived from annelid ancestors, but the 

 secondary fusion of body-segments with this head, in other words the forma- 

 tion of a secondary head, has gone on independently in the different classes 

 of the phylum Arthropoda, viz., Arachnida, Crustacea, and Tracheata 

 (including Insects and Myriapods). Judged by the number of appendages 

 (which gives an inferior limit) the head of a malacostracous Crustacean 

 consists of praestomium and 8 segments ; the head of an insect of praesto- 

 mium and 4 segments; the head of a Myriapod of praestomium and 3 

 segments; and the head of an Arachnid of praestomium and 3 segments." 

 Again, the comment of Mr. J. T. Cunningham is : — " According to Claus 

 and most modern authorities there are only 5 segments in the head of an 

 Arthropod, the eyes not counting as appendages ; and further it should be 

 noted that the second pair of antennae are wanting in Insects." 



Of course difference of opinion respecting the number of somites in the 

 head involves difference of opinion respecting the number constituting the 

 entire body, which, in the higher Arthropods, is said by some to be 19 and 

 by others 20. But those who thus differ in detail, agree in regarding all 

 the segments of head and body as homologous, and this is the essential point 

 with which we are here concerned. 



