LEADING GROUPS OF EXISTING SYSTEMS 191 



which are true Acalephs of a lower order, united into one class with 

 the Polypi. 



Fcmiily analogies. It requires little familiarity with the animal 

 kingdom to know how strong may be the resemblance between the 

 forms of animals, even when they belong to entirely different types; 

 but unless their pattern be determined by identical structural fea- 

 tures, their form certainly cannot be considered as homologous; and 

 however close the resemblance may be externally, an attempt to dis- 

 tinguish between analogical and homological forms cannot fail to 

 add precision to our zoological investigations. When, for instance, 

 the form of the Worms is compared with that of the Holothurians, 

 it should be borne in mind that in the Worms, according to the plan 

 of their structure and their homology to the other Articulates, their 

 longer diameter is the longitudinal diameter; while the longer di- 

 ameter of the Holothurians, when identified by their homologies 

 with the other Radiates, is their vertical diameter. This shows at 

 once, that however similar to one another, the form of the Holo- 

 thurians is only analogous to that of the Worms. 



The limits within which similar forms may be homologous appear 

 to be very wide and to extend beyond the limits of their respective 

 classes. The form of the Salamanders and the Lizards, for instance, is 

 certainly homological, though they are members of different classes; 

 yet similar forms within the same class are not necessarily homolo- 

 gous — for instance, the long snout of Syngnathus and that of Fistu- 

 laria, or the flat heads of Lophius and of Scaphirhynchus are only 

 remotely analogous, their structure being entirely different. The 

 forms of animals have been so imperfectly studied and the structural 

 elements which determine them so little considered, that the time 

 has hardly come yet to determine with any degree of accuracy the 

 analogies and homologies of the form of animals. Considered with 

 reference to their position, the six pairs of articulated appendages 

 which are placed upon the sides of the mouth of the horse-shoe crab 

 (Limulus) are truly homologous to the jaws of the higher Crustacea; 

 but by their form they resemble the thoracic legs of the latter; and 

 yet, as appendages to the normal rings of an Articulate, all these parts 

 are homologous. Here therefore it becomes necessary to remember 

 that while the appendages of the mouth of Limulus are only analo- 

 gous to the legs of the Decapods, as far as their form is concerned, 



