TRANSACTIONS OF THE SECTIONS. 



97 



of the animal kingdom ; for while the one has a close affinity with the little 

 freshwater Hydra, and is therefore referred to the Hydroida among the sub- 

 kingdom Crelenterata, the other is referable to the group of the Polyzoa, has its 

 immediate affinities with the Ascidians, and belongs to the great division of the 

 Molluseoida. 



We shall next take an example in which the study of development, rather than 

 of anatomy, affords the clue to the true affinities of the organism. 



Attached to the abdomen of various crabs may often be seen certain soft fleshy 

 sacs, to which the name of Saccidina has been given. They hold their place by 

 means of a branching root-like extension, which penetrates the abdomen of the 

 crab and winds itself round its intestine or dives into its liver, within which its 

 fibres ramify like the roots of a tree. 



Now the question at once presents itself, What position in the animal kingdom 

 are we to assign to this immovably-rooted sac, destitute of mouth and of almost 

 every other organ with which we are in the habit of associating the structm-e of 

 an animal ? 



Anatomy will here be powerless in helping us to arrive at a conclusion ; for the 

 dissecting-knife shows us little more than a closed sac filled with eggs, and fixed 

 by its tenacious roots in the viscera of its victim. Let us see, however, what we 

 learn from development. If some of the eggs with which the Sacculina is filled 

 be placed in conditions suited to their development, they give origin to a form as 

 different as can well be imagined from the Sacculina. It is an active, somewhat 

 oval-shaped little creature, covered with a broad dorsal shield or carapace, and 

 furnished with two pairs of strong swimming-feet, which carry long bristles, and 

 also with a pair of anterior limbs or antennae. It is, in fact, identical with a form 

 known to zoologists by the name of " Nauplius," and which has been proved to 

 be one of the young states of the Barnacle and of other lower Crustacea ; while 

 even some of the higher Crustacea have been observed to pass through a similar 

 stage. 



After a short time the Nauplius of our Sacculina changes its form ; the carapace 

 folds down on each side and assumes the shape of a little bivalve shell, while six 

 new pairs of swimming-feet are developed. The little animal continues its active 

 natatory life, and in tliis stage it is again identical in all essential points with one 

 of the .young stages of the Earnacle. 



In the mean time a remarkable change takes place in the two antennae ; they 

 become curiously branched and converted into prehensile organs. The young 

 Sacculina now looks out for the crab on which it is to spend parasiticaUy the rest 

 of its life ; it loses its bivalve shell ; the prehensile antennae take hold of its 

 victim, and penetrate the soft skin of its abdomen, in order to seek within it the 

 nutriment which is there so plentifully present ; locomotion is gone for ever, and 

 the active and symmetrical Nauplius becomes converted into the inert and shape- 

 less Sacculina. 



The nearest affinities of Sacculina are thus undoubtedly with the Barnacles, 

 which have been proved, both on anatomical and developmental grounds, to belong 

 to the gi-eat division of the Crustacea. 



A philosophical classification cannot form a single rectilineal series. 



A comparison of animals with one another having thus resulted in establishing 

 their affinities, we may arrange them into groups, some more nearly, others more 

 remotely related to one another. The various degrees and directions of affinity 

 will be expressed in every philosophical arrangement ; and as these affinities ex- 

 tend in various directions, it becomes at once apparent that no arrangement of 

 organized beings in a straight line, ascending lilve the steps of a ladder from lower 

 to higher forms, can give a true idea of the relations of such beings to one another. 

 These relations, on the contrary, can be expressed only by a ramified and complex 

 figure, which we have already compared to that of a genealogical tree. 



The following diagram will approximately express the affinities of the leading 

 groups of the animal kingdom : — 



1873. 7 



