THE EVIDENCE FROM DEVELOPMENT. 



203. 



the most part as footless, often as mouthless, and frequently as shapeless 

 organisms, attaching themselves to fishes and to other Crustaceans, 

 and living the low existence pertaining to the parasite whether of 

 higher or lower grade. There seems no wider dissimilarity, for 

 instance, between any two animals, than between the shrimp or prawn 

 (Fig. 132) and the bag-like Sacculina (Fig. 1 18), which attaches itself to- 

 the bodies of crabs. There is apparently a wide distinction between the 

 structure of a crab (Fig. 115) and a water-flea (Fig. 1 16), and still more 

 between a barnacle (Fig. 117) and a prawn (Fig. 132). Yet in the 

 classification of zoology these diverse beings are ranked as members 

 of the same class ; and development, as the great criterion of classifica- 

 tions, sanctions the arrangement. Let us endeavour to discover the 

 grounds which warrant the assertion of such near relationship. 



No fitter starting-point can be found than the development 

 of the Barnacle (Fig. 117), which, 

 attached to floating wood by its fleshy 

 peduncle or stalk, enclosed within 

 its shelly habitation, and sweeping 

 the waters with its set of feathery 

 plumes or "cirri," lives a life border- 

 ing nearly on the state of parasitism 

 itself. From the egg of the barnacle 

 and after the preliminary stages 

 of development which are common 

 in greater or less degree to the per- 

 sonal evolution of all animals 

 comes forth a little creature (Fig. 

 119), so utterly unlike its parent 

 that one might well feel disposed 

 to reject the claims of the aphorism 

 " like begets like/' so universally 

 expressive of the relation betwixt 

 parent and progeny. The body of 

 the young barnacle is triangular in 

 shape ; its anterior angles are pro- 

 truded into horn-like processes ; and 

 it possesses a mouth and digestive 

 system, a single median eye-spot, a forked tail, and three pairs of 

 feet or limbs. 



In this stage it is known as a Nanplius \ and it may be well to 

 keep the characters of this little organism in mind, since we shall 

 find them to reappear in the progeny of animals of diverse nature 

 from our Barnacle. The course of Nauplius-life lies in the direction 

 of frequent moults, and by-and-by it assumes, after a special change 

 of skin, the form of the " pupa"-barnacle (Fig. 120, B). It passes,. 



FIG. 117. BARNACLES. 



