LIFE-HISTORY OF THE HOCK BARNACLE. 5 



find more primitive forms (Fishes, &c.) which in their adult condition 

 have functional gills, and share many other features with the higher 

 vertebrate embryo. On the other hand, we find no adult forms at all 

 like a Nauplius ; the comparison, too, of the anatomy of adult Crus- 

 tacea lends no support to the view that the ancestor of the whole 

 group had the simple structure of a Nauplius, the common characters 

 of the adult members of the group giving us a picture differing 

 considerably from that of such a form. Many characters of this larva 

 are, however, so strikingly like those of adult members of the group 

 (e.g. presence of carapace, labrum, jointed appendages of special 

 character, Nauplius-eye, caudal furca, dorsal anus, &c.) as to suggest 

 that it is merely a form which has acquired early the characters 

 proper to the mature form. 



The study of embryology is, indeed, replete with questions of this 

 kind. We may at first be inclined to chafe at the difficulties Ave 

 meet, and to wish that the life-history presented us with a perfect 

 picture of the history of the race, but it must be remembered that 

 the reconstruction of the genealogical tree is not the only task of 

 biology. The study of the special adaptations of larvae to their 

 surroundings is one of great importance : the adaptive relations of 

 the adults have, indeed, formed a branch of natural history, but 

 comparatively little has been done in viewing the young forms from 

 this teleological point of view. We ought to have as accurate a 

 knowledge of the use and adaptation of the parts of larva?, as we aim 

 at getting in the case of the adults. Interesting results have, indeed, 

 been obtained in this direction, and amongst these I may specially 

 mention Prof. Miall's late investigations on aquatic Insect larva?, but 

 a great field is still open to anyone who will undertake it. 



Explanation of Plate I, Vol, II. 

 Life-History of Balanus (Part II). 



Fig. 21. Tergum of young Balanus balanoides considerably mag- 

 nified to show the punctures on the shell. 



Fig. 22. Last Nauplius-stage (VI) of a species of Balanus obtained, 

 together with the Cypris-stages of B. balanoides, from 

 Jersey. In addition to the three pairs of Nauplius- 

 appendages and first pair of maxilla?, the remaining 

 appendages can be seen beneath the cuticle. The com- 

 pound eyes can be seen through the antennules on each 

 side of the Nauplius-eye : through the basal part of the 

 labrum can be seen the oesophagus suspended by simple 

 unstriated muscular fibres. The position of the gnatho- 



